Ion implantation ion source, system and method

ABSTRACT

Various aspects of the invention provide improved approaches and methods for efficiently: Vaporizing decaborane and other heat-sensitive materials via a novel vaporizer and vapor delivery system; Delivering a controlled, low-pressure drop flow of vapors, e.g. decaborane, into the ion source; 
         Ionizing the decaborane into a large faction of B 10 H x   + ; Preventing thermal dissociation of decaborane;    Limiting charge-exchange and low energy electron-induced fragmentation of B 10 H x   + ; Operating the ion source without an arc plasma, which can improve the emittance properties and the purity of the beam; Operating the ion source without use of a strong applied magnetic field, which can improve the emittance properties of the beam; Using a novel approach to produce electron impact ionizations without the use of an arc discharge, by incorporation of an externally generated, broad directional electron beam which is aligned to pass through the ionization chamber to a thermally isolated beam dump; Providing production-worthy dosage rates of boron dopant at the wafer; Providing a hardware design that enables use also with other dopants, especially using novel hydride, dimer-containing, and indium- or antimony-containing temperature-sensitive starting materials, to further enhance the economics of use and production worthiness of the novel source design and in many cases, reducing the presence of contaminants; Matching the ion optics requirements of the installed base of ion implanters in the field; Eliminating the ion source as a source of transition metals contamination, by using an external and preferably remote cathode and providing an ionization chamber and extraction aperture fabricated of non-contaminating material, e.g. graphite, silicon carbide or aluminum; Enabling retrofit of the new ion source into the ion source design space of existing Bernas source-based ion implanters and the like or otherwise enabling compatibility with other ion source designs; Using a control system in retrofit installations that enables retention of the installed operator interface and control techniques with which operators are already familiar; Enabling convenient handling and replenishment of the solid within the vaporizer without substantial down-time of the implanter; Providing internal adjustment and control techniques that enable, with a single design, matching the dimensions and intensity of the zone in which ionization occurs to the beam line of the implanter and the requirement of the process at hand; Providing novel approaches, starting materials and conditions of operation that enable the making of future generations of semiconductor devices and especially CMOS source/drains and extensions, and doping of silicon gates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of and benefit of U.S. ProvisionalPatent Application No. 60/170,473 filed on Dec. 13, 1999, U.S.Provisional Patent Application No. 60/250,080 filed on Nov. 30, 2000 andPCT Patent Application No. PCT/US00/33786 filed on Dec. 13, 2000.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention provides production-worthy ion sources and methods capableof using new source materials, in particular, heat-sensitive materialssuch as decaborane (B₁₀H₁₄), and hydrides and dimer-containing compoundsnovel to the ion implantation process, to achieve new ranges ofperformance in the commercial ion implantation of semiconductor wafers.The invention enables shallower, smaller and higher densities ofsemiconductor devices to be manufactured, particularly in ComplementaryMetal-Oxide Semiconductor (CMOS) manufacturing. In addition to enablinggreatly enhanced operation of new ion implanter equipment in themanufacture of semiconductor devices, the invention enables the new ionsource to be retrofit into the existing fleet of ion implanters withgreat capital cost savings. Embodiments of the invention uniquelyimplant decaborane and the other dopant materials in particularly pureion beams, enabling a wide range of the needs of a fabrication facilityto be met. Various novel constructional, operational and processfeatures that contribute to the cost-effectiveness of the new technologyare applicable as well to prior technology of the industry.

2. Description of the Prior Art

As is well known, ion implantation is a key technology in themanufacture of integrated circuits (ICs). In the manufacture of logicand memory ICs, ions are implanted into silicon or GaAs wafers to formtransistor junctions, and to dope the well regions of the p-n junctions.By selectively controlling the energy of the ions, their implantationdepth into the target wafer can be selectively controlled, allowingthree-dimensional control of the dopant concentrations introduced by ionimplantation. The dopant concentrations control the electricalproperties of the transistors, and hence the performance of the ICs. Anumber of dopant feed materials have previously been used, including As,Ar, B, Be, C, Ga, Ge, In, N, P, Sb and Si. For those species which areof solid elemental form, many are obtainable in gaseous molecular form,such as fluoride compounds that are ionizable in large quantities atsignificantly elevated temperatures. The ion implanter is amanufacturing tool which ionizes the dopant-containing feed materials,extracts the dopant ions of interest, accelerates the dopant ions to thedesired energy, filters away undesired ionic species, and thentransports the dopant ions of interest to the wafer at the appropriateenergy for impact upon the wafer. The presence in the implanter ofcertain elements, such as the disassociated element fluorine, isdetrimental to the implanted wafers, but, despite such drawbacks, traceamounts of such contaminants have been tolerated in many contexts, inthe interest of achieving production-worthy throughput volume. Lowercontaminant levels from what is now achievable is desired.

In a complex relationship, overall, a number of variables must becontrolled in order to achieve a desired implantation profile for agiven ion implantation process:

-   -   The nature of the dopant feed material (e.g., BF₃ gas)    -   Dopant ion species (e.g., B⁺)    -   Ion energy (e.g., 5 keV)    -   Chemical purity of the ion beam (e.g., <0.01% energetic        contaminants)    -   Isotopic purity of the ion beam (e.g., ability to discriminate        between ¹¹³In and ¹¹⁵In)    -   Energy purity of the ion beam (e.g., <2% full width at half        maximum, i.e. FWHM)    -   Angular divergence and spatial extent, of the beam on the wafer    -   Total dose (e.g., 10¹⁵ atoms/cm²) implanted on the wafer    -   Uniformity of the dose (e.g., ±1% variation in the implanted        density over the total wafer surface area).

These variables affect the electrical performance, minimummanufacturable size and maximum manufacturable density of transistorsand other devices fabricated through ion implantation.

A typical commercial ion implanter is shown in schematic in FIG. 1. Theion beam I is shown propagating from the ion source 42 through atransport (i.e. “analyzer”) magnet 43, where it is separated along thedispersive (lateral) plane according to the mass-to-charge ratio of theions. A portion of the beam is focused by the magnet 43 onto a massresolving aperture 44. The aperture size (lateral dimension) determineswhich mass-to-charge ratio ion passes downstream, to ultimately impactthe target wafer 55, which typically may be mounted on a spinning disk45. The smaller the mass resolving aperture 44, the higher the resolvingpower R of the implanter, where R=M/ΔM (M being the nominalmass-to-charge ratio of the ion and ΔM being the range of mass-to-chargeratios passed by the aperture 44). The beam current passing aperture 44can be monitored by a moveable Faraday detector 46, whereas a portion ofthe beam current reaching the wafer position can be monitored by asecond Faraday detector 47 located behind the disk 45. The ion source 42is biased to high voltage and receives gas distribution and powerthrough feedthroughs 48. The source housing 49 is kept at high vacuum bysource pump 50, while the downstream portion of the implanter islikewise kept at high vacuum by chamber pump 51. The ion source 42 iselectrically isolated from the source housing 49 by dielectric bushing52. The ion beam is extracted from the ion source 42 and accelerated byan extraction electrode 53. In the simplest case (where the sourcehousing 49, implanter magnet 43, and disk 45 are maintained at groundpotential), the final electrode of the extraction electrode 53 is atground potential and the ion source is floated to a positive voltageV_(a), where the beam energy E=qV_(a) and q is the electric charge perion. In this case, the ion beam impacts the wafer 55 with ion energy E.In other implanters, as in serial implanters, the ion beam is scannedacross a wafer by an electrostatic or electromagnetic scanner, witheither a mechanical scan system to move the wafer or another suchelectrostatic or electromagnetic scanner being employed to accomplishscanning in the orthogonal direction.

A part of the system of great importance in the technology of ionimplantation is the ion source. FIG. 2 shows diagrammatically the“standard” technology for commercial ion sources, namely the “EnhancedBernas” arc discharge ion source. This type of source is commonly thebasis for design of various ion implanters, including high current, highenergy, and medium current ion implanters. The ion source a is mountedto the vacuum system of the ion implanter through a mounting flange bwhich also accommodates vacuum feedthroughs for cooling water,thermocouples, dopant gas feed, N₂ cooling gas, and power. The dopantgas feed c feeds gas, such as the fluorides of a number of the desireddopant species, into the arc chamber d in which the gas is ionized. Alsoprovided are dual vaporizer ovens e, f inside of the mounting flange inwhich solid feed materials such as As, Sb₂O₃, and P may be vaporized.The ovens, gas feed, and cooling lines are contained within a watercooled machined aluminum block g. The water cooling limits thetemperature excursion of the aluminum block g while the vaporizers,which operate between 100C and 800C, are active, and also counteractsradiative heating by the arc chamber d when the ion source is active.The arc chamber d is mounted to, but designedly is in poor thermalcontact with, the aluminum block g. The ion source a employs an arcdischarge plasma, which means that it operates by sustaining within adefined chamber volume a generally narrow continuous electric arcdischarge between hot filament cathode h, residing within the arcchamber d, and the internal walls of the arc chamber d. The arc producesa narrow hot plasma comprising a cloud of primary and secondaryelectrons interspersed with ions of the gas that is present. Since thisarc can typically dissipate in excess of 300 W energy, and since the arcchamber d cools only through radiation, the arc chamber in such Bernasion sources can reach a temperature of 800C during operation.

The gas is introduced to arc chamber d through a low conductance passageand is ionized through electron impact with the electrons dischargedbetween the cathode h and the arc chamber d and, as well, by the manysecondary electrons produced by the arc discharge. To increaseionization efficiency, a substantial, uniform magnetic field i isestablished along the axis joining the cathode h and an anticathode j byexternally located magnet coils, 54 as shown in FIG. 1. This providesconfinement of the arc electrons, and extends the length of their paths.The anticathode j (sometimes referred to as a “repeller”) located withinthe arc chamber d but at the end opposite the cathode h is typicallyheld at the same electric potential as the cathode h, and serves toreflect the arc electrons confined by the magnetic field i back towardthe cathode h, from which they are repelled back again, the electronstraveling repeatedly in helical paths. The trajectory of thethus-confined electrons results in a cylindrical plasma column betweenthe cathode h and anticathode j. The arc plasma density within theplasma column is typically high, on the order of 10¹² per cubiccentimeter; this enables further ionizations of the neutral and ionizedcomponents within the plasma column by charge-exchange interactions, andalso allows for the production of a high current density of extractedions. The ion source a is held at a potential above ground (i.e., abovethe potential of the wafer 55) equal to the accelerating voltage V_(a)of the ion implanter: the energy, E, of the ions as they impact thewafer substrate is given by E=qV_(a), where q is the electric charge perion.

The cathode h of such a conventional Bernas arc discharge ion source istypically a hot filament or an indirectly-heated cathode whichthermionically emits electrons when heated by an external power supply.It and the anticathode are typically held at a voltage V_(c) between 60Vand 150V below the potential of the ion source body V_(a). Once an arcdischarge plasma is initiated, the plasma develops a sheath adjacent theexposed surface of the cathode h within chamber d. This sheath providesa high electric field to efficiently extract the thermionic electroncurrent for the arc; high discharge currents (e.g., up to 7A) can beobtained by this method.

The discharge power P dissipated in the arc chamber is P=D V_(c),typically hundreds of watts. In addition to the heat dissipated by thearc, the hot cathode h also transfers power to the walls of the arcchamber d. Thus, the arc chamber d provides a high temperatureenvironment for the dopant arc plasma, which boosts ionizationefficiency relative to a cold environment by increasing the gas pressurewithin the arc chamber d, and by preventing substantial condensation ofdopant material on the hot chamber walls.

If the solid source vaporizer ovens e or f of the Bernas arc dischargeion source are used, the vaporized material feeds into the arc chamber dwith substantial pressure drop through narrow vaporizer feeds k and l,and into plenums m and n. The plenums serve to diffuse the vaporizedmaterial into the arc chamber d, and are at about the same temperatureas the arc chamber d. Radiative thermal loading of the vaporizers by thearc chamber also typically prevents the vaporizers from providing astable temperature environment for the solid feed materials containedtherein below about 200C. Thus, typically, only solid dopant feedmaterials that both vaporize at temperatures >200C and decompose attemperatures >800C (the temperature of the walls of the ionizationchamber of a typical Bernas source) can be successfully vaporized andintroduced by this method.

A very significant problem which currently exists in the ionimplantation of semiconductors is the limitation of production-worthyion implantation implanters that prevents effective implanting of dopantspecies at low (e.g., sub-keV) energies at commercially desired rates.One critically important application which utilizes low-energy dopantbeams is the formation of shallow transistor junctions in CMOSmanufacturing. As transistors shrink in size to accommodate moretransistors per IC according to a vital trend, the transistors must beformed closer to the surface of the target wafer. This requires reducingthe velocity, and hence the energy, of the implanted ions, so that theydeposit at the desired shallow level. The most critical need in thisregard is the implantation of low-energy boron, a p-type dopant, intosilicon wafers. Since boron atoms have low mass, at a given energy forwhich the implanter is designed to operate, they must have highervelocity and will penetrate deeper into the target wafer than otherp-type dopants; therefore there is a need for boron to be implanted atlower energies than other species.

Ion implanters are relatively inefficient at transporting low-energy ionbeams due to space charge within the ion beam, the lower the energy, thegreater the problem. The space charge in low energy beams causes thebeam cross-section area (i.e. its “profile”) to grow larger as the ionsproceed along the beam line (there is “beam blow-up”). When the beamprofile exceeds the profile for which the implanter's transport opticshave been designed, beam loss through vignetting occurs. For example, at500 eV transport energy, many ion implanters currently in use cannottransport enough boron beam current to be commercially efficient inmanufacturing; i.e., the wafer throughput is too low because of lowimplantation dose rate. In addition, known ion sources rely on theapplication of a strong magnetic field in the source region. Since thismagnetic field also exists to some extent in the beam extraction regionof the implanter, it tends to deflect such a low-energy beam andsubstantially degrade the emittance properties of the beam, whichfurther can reduce beam transmission through the implanter.

An approach has been proposed to solve the problem of low-energy boronimplantation: molecular beam ion implantation. Instead of implanting anion current I of atomic B⁺ ions at an energy E, a decaborane molecularion, B₁₀H_(x) ⁺, is implanted at an energy 10×E and an ion current of0.10×I. The resulting implantation depth and dopant concentration (dose)of the two methods have been shown to be generally equivalent, with thedecaborane implantation technique, however, having significant potentialadvantages. Since the transport energy of the decaborane ion is tentimes that of the dose-equivalent boron ion, and the ion current isone-tenth that of the boron current, the space charge forces responsiblefor beam blowup and the resulting beam loss can potentially be muchreduced relative to monatomic boron implantation.

While BF₃ gas can be used by conventional ion sources to generate B⁺ions, decaborane (B₁₀H₁₄) must be used to generate the decaborane ionB₁₀H_(x) ⁺. Decaborane is a solid material which has a significant vaporpressure, on the order of 1 Torr at 20C, melts at 100C, and decomposesat 350C. To be vaporized through preferred sublimination, it musttherefore be vaporized below 100C, and it must operate in aproduction-worthy ion source whose local environment (walls of theionization chamber and components contained within the chamber) is below350C to avoid decomposition. In addition, since the B₁₀H₁₄ molecule isso large, it can easily disassociate (fragment) into smaller components,such as elemental boron or diborane (B₂H₆), when subject tocharge-exchange interactions within an arc discharge plasma, hence it isrecognized that conventionally operated Bernas arc plasma sources cannot be employed in commercial production, and that ionization should beobtained primarily by impact of primary electrons. Also, the vaporizersof current ion sources cannot operate reliably at the low temperaturesrequired for decaborane, due to radiative heating from the hot ionsource to the vaporizer that causes thermal instability of themolecules. The vaporizer feed lines k, I can easily become clogged withboron deposits from decomposed vapor as the decaborane vapor interactswith their hot surfaces. Hence, the present production-worthy implanterion sources are incompatible with decaborane ion implantation. Priorefforts to provide a specialized decaborane ion source have not met themany requirements of production-worthy usage.

More broadly, there are numerous ways in which technology that has beencommon to the industry has had room for improvement. Cost-effectivefeatures, presented here as useful in implementing the new technology,are applicable to implementation of the established technology as well.

SUMMARY OF THE INVENTION

Various aspects of the invention provide improved approaches and methodsfor efficiently:

-   -   Vaporizing decaborane and other heat-sensitive materials via a        novel vaporizer and vapor delivery system;    -   Delivering a controlled, low-pressure drop flow of vapors, e.g.        decaborane, into the ion source;    -   Ionizing the decaborane into a large fraction of B₁₀H_(x) ⁺;    -   Preventing thermal dissociation of decaborane;    -   Limiting charge-exchange and low energy electron-induced        fragmentation of B₁₀H_(x) ⁺;    -   Operating the ion source without an arc plasma, which can        improve the emittance properties and the purity of the beam;    -   Operating the ion source without use of a strong applied        magnetic field, which can improve the emittance properties of        the beam;    -   Using a novel approach to produce electron impact ionizations        without the use of an arc discharge, by incorporation of an        externally generated, broad directional electron beam which is        aligned to pass through the ionization chamber to a thermally        isolated beam dump;    -   Providing production-worthy dosage rates of boron dopant at the        wafer;    -   Providing a hardware design that enables use also with other        dopants, especially using novel hydride, dimer-containing, and        indium- or antimony-containing temperature-sensitive starting        materials, to further enhance the economics of use and        production worthiness of the novel source design and in many        cases, reducing the presence of contaminants;    -   Matching the ion optics requirements of the installed base of        ion implanters in the field;    -   Eliminating the ion source as a source of transition metals        contamination, by using an external and preferably remote        cathode and providing an ionization chamber and extraction        aperture fabricated of non-contaminating material, e.g.        graphite, silicon carbide or aluminum;    -   Enabling retrofit of the new ion source into the ion source        design space of existing Bernas source-based ion implanters and        the like or otherwise enabling compatibility with other ion        source designs;    -   Using a control system in retrofit installations that enables        retention of the installed operator interface and control        techniques with which operators are already familiar;    -   Enabling convenient handling and replenishment of the solid        within the vaporizer without substantial down-time of the        implanter;    -   Providing internal adjustment and control techniques that        enable, with a single design, matching the dimensions and        intensity of the zone in which ionization occurs to the beam        line of the implanter and the requirement of the process at        hand;    -   Providing novel approaches, starting materials and conditions of        operation that enable the making of future generations of        semiconductor devices and especially CMOS source/drains and        extensions, and doping of silicon gates;    -   And generally, providing features, relationships and methods        that achieve production-worthy ionization of decaborane and        numerous other dopant feed materials many of which are novel to        ion implantation, to meet the practical needs of fabrication        facilities.

Embodiments of the present invention can enhance greatly the capabilityof new ion implantation systems and can provide a seamless andtransparent upgrade to end-users' existing implanters.

In particular, aspects of the invention are compatible with current ionimplantation technology, such that an ion source constructed accordingto the invention can be retrofitted into the existing fleet of ionimplanters currently installed in expensive fabrication plants.Embodiments of the invention are (1) constructed, sized and arrangedsuch that they fit into the existing ion source space of commercialimplanters, and 2) employ a novel control system for the ion sourcewhich can physically replace the existing ion source controller, withoutfurther modification of the implanter controls and qualified productiontechniques.

According to one aspect of the invention, an ion source capable ofproviding ions in commercial ion current levels to the ion extractionsystem of an ion implanter is provided, the ion source comprising anionization chamber defined by walls enclosing an ionization volume,there being an ion extraction aperture in a side wall of the ionizationchamber, the aperture having a length and width sized and arranged toenable the ion current to be extracted from the ionization volume by theextraction system. The invention features a broad beam electron gunconstructed, sized and arranged with respect to the ionization chamberto direct an aligned beam of primary electrons through the ionizationchamber to a beam dump maintained at a substantial positive voltagerelative to the emitter voltage of the electron beam gun. Preferably thebeam dump is thermally isolated from the ionization chamber orseparately cooled. The axis of the beam path of the primary electronsextends in a direction generally adjacent to the aperture, the electronbeam having a dimension in the direction corresponding to the directionof the width of the extraction aperture that is about the same as orlarger than the width of the aperture, a vaporizer arranged to introducee.g. decaborane vapor to the ionization volume, and a control systemenables control of the energy of the primary electrons so thatindividual vapor molecules can be ionized principally by collisions withprimary electrons from the electron gun.

In preferred embodiments the electron gun is mounted on a support thatis thermally isolated from the walls of the ionization chamber.

According to another aspect of the invention, an ion source capable ofproviding ions of decaborane in commercial ion current levels to the ionextraction system of an ion implanter is provided, the ion sourcecomprising an ionization chamber defined by walls enclosing anionization volume, there being an ion extraction aperture in a side wallof the ionization chamber, arranged to enable the ion current to beextracted from the ionization volume by an extraction system, anelectron gun mounted on a support that is outside of and thermallyisolated from the walls of the ionization chamber, and constructed,sized and arranged with respect to the ionization chamber to direct abroad beam of primary electrons through the ionization chamber to a beamdump maintained at a substantial positive voltage relative to theemitter voltage of the electron beam gun, the beam dump being locatedoutside of, and thermally isolated from, the ionization chamber, thebeam path of the primary electrons extending in a direction adjacent tothe ion extraction aperture, a passage arranged to introduce vapor orgas of a selected material to the ionization volume, and a controlsystem enabling control of the energy of the primary electrons so thatthe material can be ionized.

According to another aspect of the invention, an ion source capable ofproviding ions in commercial ion current levels to the ion extractionsystem of an ion implanter is provided, the ion source comprising anionization chamber defined by walls enclosing an ionization volume,there being an extraction aperture in a side wall of the ionizationchamber that is arranged to enable the ion current to be extracted fromthe ionization volume by the extraction system, an electron gun mountedon a support that is outside of and thermally isolated from the walls ofthe ionization chamber, and constructed, sized and arranged with respectto the ionization chamber to direct a broad beam of primary electronsthrough the ionization chamber to a beam dump maintained at asubstantial positive voltage relative to the emitter voltage of theelectron beam gun, the electron beam gun comprising a heated electronemitting surface of predetermined size followed by electron opticalelements that enlarge the beam in the ionization chamber relative to thesize of the emitting surface of the electron gun, the beam path of theprimary electrons extending in a direction adjacent to the ionextraction aperture, a passage arranged to introduce vapor or gas of aselected material to the ionization volume, and a control systemenabling control of the energy of the primary electrons so that thematerial can be ionized.

Preferred embodiments of these and other aspects of the invention haveone or more of the following features:

A vaporizer is incorporated into the ion source assembly in closeproximity to the ionization chamber and communicating with it through ahigh conductance, preferably along a line of sight path, and isconstructed to be controllable over part or all of the range of 20C to200C.

The beam dump has an electron-receiving surface larger than thecross-section of the electron beam entering the ionization chamber.

The electron gun produces a generally collimated beam, in manyinstances, preferably the electron gun being generally collimated whiletransiting the ionization chamber.

The beam dump is mounted on a dynamically cooled support, preferably awater-cooled support.

The electron gun is mounted on a dynamically cooled support, preferably,a water-cooled support.

The electron gun cathode is disposed in a position remote from theionization chamber.

The volume occupied by the electron gun cathode is evacuated by adedicated vacuum pump.

The ion source electron gun includes a cathode and variable electronoptics that shape the flow of electrons into a beam of selectedparameters, including a general dispersion of the electrons, and aprofile matched to the extraction aperture, preferably in many cases theelectrons being in a collimated beam within the ionization chamber.

The electron gun comprises a high transmission electron extraction stagecapable of extracting at least the majority of electrons from an emitterof the gun, the extraction stage followed by a collimator and furtherelectron optic elements, in preferred embodiments the further electronoptics comprising an electron zoom lens or electron optics constructedto have the capability to vary the energy and at least one magnificationparameter of the electron beam, preferably both linear and angularmagnification of the beam and in preferred embodiments the electronoptics comprising a five or more element zoom lens.

The ion source is constructed, sized and arranged to be retrofit into apre-existing ion implanter, into the general space occupied by theoriginal ion source for which the implanter was designed.

The ion source is constructed and arranged to cause the electron beam tohave a profile matched to the opening of the ion extraction aperture,preferably the cross-section being generally rectangular.

The electron beam gun of the ion source is an elongated electron gun, incertain embodiments the length of the gun being longer than the lengthof the ionization path length in the ionization chamber, preferably,e.g. for retrofit installations, the principal direction of theelongated electron gun being arranged generally parallel to thedirection in which the ion beam is extracted from the ionizationchamber, and an electron mirror is arranged to divert the electron beamto a transverse direction to pass through the ionization volume. In thisand other embodiments, preferably the cathode of the elongated electronbeam gun is a uniform emitting surface sized smaller than the maximumcross-section of the electron beam passing through the ionizationchamber, and the electron optics include optics arranged to expand theelectron beam before it enters the ionization chamber. In variousembodiments some of the optics precede the mirror or are downstream ofthe mirror, and the optics are constructed to vary angular as well aslinear magnification. Preferably these optics comprise a zoom control toenable variation of the electron energy of the beam.

The control system includes a circuit for measuring the current and theintensity of the beam dump.

The ion source electron beam gun is constructed to operate with avoltage drop relative to the walls of the ionization chamber betweenabout 20 and 300 or 500 electron volts; preferably, to ionizedecaborane, the voltage drop being between 20 and 150 electron volts,higher voltages being useful for providing double charges on selectedimplant species or for providing ionizing conditions for other feedmaterials.

For use with a previously existing ion implanter designed for use with aBernas arc discharge source having a directly or indirectly heatedcathode, the control system includes an operator control screencorresponding to the screen used for the Bernas source, and a translatoreffectively translates arc current control signals to control signalsfor the electron gun.

The ionization chamber is in thermal continuity with the vaporizer, orwith a temperature control device.

The vaporizer for decaborane includes a temperature control system, andthe ionization chamber is in thermal continuity with the vaporizer,preferably the ionization chamber is defined within a conductive blockdefining a heat sink that is in thermal continuity with the vaporizer,preferably, the conductive block being in thermal continuity with thevaporizer via one or more conductive gaskets, including a gasket atwhich the vaporizer may be separated from the remainder of the assembly.

The ionization chamber is defined by a removable block disposed in heattransfer relationship to a temperature controlled mounting block,preferably the removable block comprised of graphite, silicon carbide oraluminum.

The ion source includes a mounting flange for joining the ion source tothe housing of an ion implanter, the ionization chamber being located onthe inside of the mounting flange and the vaporizer being removablymounted to the exterior of the mounting flange via at least oneisolation valve which is separable from the mounting flange with thevaporizer, enabling the vaporizer charge volume to be isolated by thevalve in closed position during handling, preferably there being twoisolation valves in series, one unified with and transportable with aremoved vaporizer unit, and one constructed to remain with and isolatethe remainder of the ion source from the atmosphere. In certainpreferred embodiments, two such vaporizers are provided, enabling one tobe absent, while being charged or serviced, while the other operates, orenabling two different materials to be vaporized without maintenance ofthe ion source, or enabling additional quantities of the same materialsto be present to enable a protracted implant run.

Opposite walls of the ionization chamber corresponding respectively tothe electron beam gun and the beam dump have ports through whichelectrons pass enroute from the electron beam gun to the beam dump, thespaces in the vicinity of the ports being surrounded by housing andcommunicating with a vacuum system.

The ion source includes a gas inlet via into which compounds such asarsine, phosphene, germane and silane gas can be introduced to theionization chamber for ionization.

The extraction aperture of the ionization chamber, for e.g. high currentmachines, is about 50 mm or more in length and at least about 3.5 mm inwidth, and the transverse cross sectional area of the electron beam isat least about 30 square mm, preferably, e.g. for decaborane in highcurrent machines, the cross-sectional area of the beam being at leastabout 60 square mm. For a medium current ion implanter preferably theextraction aperture is at least 15 mm in length and at least about 1.5mm in width, and the transverse cross sectional area of the electronbeam is at least about 15 square millimeters. In many medium currentimplanters, the extraction aperture can be sized 20 mm long by 2 mmwide, in which case the cross-sectional area of the electron beam can bereduced to a minimum of about 20 square mm.

An ion implantation system is provided comprising an ion implanterdesigned for a first ion source occupying a general design volume, and asecond ion source of any of the novel types described above isoperatively installed in that volume, preferably the electron gun beingof elongated form, having its principal direction arranged parallel tothe direction the ion beam is extracted from the ionization chamber, andan electron mirror is arranged to divert the electron beam to atransverse direction to pass through the ionization volume. In this andother embodiments of an ion implantation system, preferably the cathodeis sized smaller than the maximum cross-section of the electron beampassing through the ionization chamber, and the electron optics includeoptics arranged to expand the electron beam before it enters theionization chamber, preferably these optics being associated with a zoomcontrol to enable controlled variation of the electron energy.

The invention also features methods of employing apparatus having thevarious features described to ionize decaborane, the mentioned hydridesand other temperature-sensitive materials including indium-, antimony-,and dimer-containing compounds. The methods include using the variousmethods of control that are described in the preceding description andin the following text.

In particular, the invention includes the methods described ofgenerating the electron beam, accelerating and collimating the beam,controlling its transverse profile and its energy, and causing it totransmit the ionization chamber to create the desired ions while keepingthe ionization chamber cool. It also includes the methods of vaporizingthe solid materials and cooling the ionization chamber with thevaporizer heat control system as well as controlling the vaporproduction of the vaporizer by pressure control or by a dual temperatureand pressure control that is for instance capable of adjusting for thedecreasing volume of the feed material as operation proceeds.

Particular aspects of the invention feature methods of providing ionsduring ion implantation comprising introducing material comprising a gasor heated vapor to a chamber enclosing an ionization volume, the chamberhaving an extraction aperture, and passing through the ionization volumeadjacent the aperture a broad beam of electrons. According to one aspectof the invention, the broad beam is aligned with a beam dump that isthermally isolated from the chamber, the energy of the electrons beingselected to ionize the material. According to another aspect, the energyand magnification of the electron beam are controlled with electron zoomoptics to ionize the material. According to another aspect, the beam isformed and the energy of the electrons is controlled by successivelyaccelerating and decelerating the electrons. In preferred embodiments ofthese aspects the broad electron beam is emitted from a heated emittersurface that is remote from and thermally isolated from the ionizationchamber; electrons from an emitter surface are accelerated, collimatedand passed through beam-expanding optics before passing through theionization chamber, and, for vaporizing decaborane, the method includesintroducing the decaborane vapors to the ionization chamber, andcontrolling the broad electron beam parameters to ionize the decaboraneand produce a decaborane current, or the method includes introducing tothe ionization chamber a hydride of a desired species, and ionizing thehydride, in preferred embodiments the hydride being arsine or phospheneor germane or silane or diborane. In other preferred methods, anindium-containing compound is employed including introducing the indiumcompound vapors to the ionization chamber, and controlling the broadelectron beam parameters to ionize the indium compound and produce anindium ion current, preferably the compound being trimethyl indium. Instill other preferred methods, a compound containing antimony isemployed including introducing the antimony compound vapors to theionization chamber, and controlling the broad electron beam parametersto ionize the antimony compound and produce an antimony ion current,preferably the compound being antimony oxide (Sb₂O₅). Otherdimer-containing compounds described below are also useful, both forproducing dimer ions and monomer ions. In the various methods preferablya beam dump is employed to receive the electron beam after it transmitsthe ionization volume, including maintaining the beam dump thermallyisolated from the chamber and at a voltage potential at least as high asthat of the chamber.

In some instances a magnetic field is applied to constrain the electronbeam, e.g. to counteract space-charge effects. In some instances, forcertain compounds, preferably the process described is converted to areflex ionization mode by changing the potential of the beam dump to asubstantially lower potential than the walls of the ionization chamberto act as an electron-repelling anticathode, in certain cases the methodincluding applying a magnetic field parallel to the electron beam, orcontinuing to cool the walls of the ionization chamber while operatingin reflex mode.

The invention also features the methods of retrofitting the new ionsource into the existing fleet of ion implanters, and of controlling theion source by means of the operator interface of the arc plasma ionsource that it replaces.

Also, the invention features methods of conducting entire ionimplantation processes using the equipment and controls described toform semiconductor devices, in particular shallow source/drains andextensions, and doping of the silicon gates in CMOS fabrication.

In addition, the invention features methods and apparatus for dual modeoperation, both a broad E-Beam mode with the beam aligned with a beamdump at positive potential and a reflex mode, in which the dump isconverted to a repeller (anticathode) with optional use of a confiningmagnetic field, advantageously both conducted with cooled walls toionize materials such as hydrides that disassociate with elevatedtemperatures.

In the method employing a broad electron beam directed to a beam dump,in certain cases the invention features applying a magnetic field toconstrain the electron beam.

According to another aspect of the invention, an ion source is providedhaving a member whose surface is exposed to contact of a dopant feedmaterial, including gases, vapors or ions thereof, the relationship ofthe contact being such that condensation or molecular dissociation willoccur if the temperature of the surface of the member is not within adesired operational range, the member being disposed in conductive heattransfer relationship with a second member, the temperature of which isactively controlled. The temperature of the second member can bedetermined by water-cooling the member with de-ionized water of a giventemperature. The second member can be associated with a thermoelectriccooling unit associated with a control system that can activate the unitto maintain the temperature of the surface within said operationalrange. In some cases a heater element is included which is arranged tocooperate with the cooling unit to maintain the second member at atemperature. In certain embodiments the cooling unit has a surface whichforms a thermally conductive interface with an opposed surface of themember. In certain preferred embodiments a conductive gas fills gaps atan interface in the conductive path under conditions in which the gasmolecules act to transfer heat across the interface by conduction,preferably the conductive gas being fed into channels formed in at leastone of the surfaces across which the thermal heat conduction is tooccur.

The invention also features a control system for the vaporizer whichincludes an ionization gauge sensitive to a pressure related to apressure within the ionization chamber.

Another aspect of the invention is an ion source which includes anaccel-decel electron gun arranged to project a beam of electrons throughan ionization chamber to ionize gas or vapors in a region adjacent anextraction aperture.

Preferred embodiments of this aspect have one or more of the followingfeatures:

A magnetic coil is disposed outside of the ionization chamber, theelectron gun is mounted concentrically with the coil, such that theemission axis of the electron gun is aligned to emit electrons into theionization chamber and the coil, when energized, provides a magneticfield which limits space charge expansion of the electron beam as ittransmits the ionization chamber.

The volume occupied by the electron gun cathode is evacuated by adedicated vacuum pump.

A beam dump at a positive voltage is aligned to receive electrons of thebeam that transmit the ionization chamber.

This accel-decel electron gun is disposed outside of an ionizationchamber, the electron gun mounted such that the emission axis of theelectron gun is aligned to emit electrons into the ionization chamber.

The accel-decel gun has an electron zoom lens. The accel-decel gun iscomprised of a high-transmission extraction stage followed by a focusinglens having at least two elements followed by a relatively short,strongly-focusing lens which acts to decelerate the electron beamentering the ionization chamber, preferably the short lens being amulti-aperture lens comprising a series of at least two conductingplates each having an aperture, the voltage on the plates being ofrespectively decreasing values to decelerate the electrons.

The beam deceleration stage of the electron gun focuses the beam in theionization chamber at a point near mid-length of an elongated aperture,past which the electron beam passes.

Other aspects and detailed features of the invention will be apparentfrom the drawings, the following description of preferred embodiments,and from the claims and

GENERAL DESCRIPTION

An embodiment of an ion source incorporating various aspects of theinvention is composed of i) a vaporizer, ii) a vaporizer valve, iii) agas feed, iv) an ionization chamber, v) an electron gun, ii) a cooledmounting frame, and vii) an ion exit aperture. Included are means forintroducing gaseous feed material into the ionization chamber, means forvaporizing solid feed materials and introducing their vapors into theionization chamber, means for ionizing the introduced gaseous feedmaterials within the ionization chamber, and means for extracting theions thus produced from an ion exit aperture adjacent to the ionizationregion. In addition, means for accelerating and focusing the exitingions are provided. The vaporizer, vaporizer valve, gas feed, ionizationchamber, electron gun, cooled mounting frame, and ion exit aperture areall integrated into a single assembly in preferred embodiments of thenovel ion source. I will describe each of these features.

Vaporizer: The vaporizer is suitable for vaporizing solid materials,such as decaborane (B₁₀H₁₄) and TMI (trimethyl indium), which haverelatively high vapor pressures at room temperature, and thus vaporizeat temperatures below 100C. The temperature range between roomtemperature and 100C is easily accommodated by embodiments in which thevaporizer is directly associated with a water heat transfer medium,while other preferred arrangements accommodate novel material whichproduce significant vapor pressures in the range up to 200C. Forexample, solid decaborane has a vapor pressure of about 1 Torr at 20C.Most other implant species currently of interest in the ion implantationof semiconductors, such as As, P, Sb, B, C, Ar, N, Si, and Ge areavailable in gaseous forms. However, B₁₀ and In are not, but can bepresented in vapors from solid decaborane and TMI. The vaporizer of anembodiment of the invention is a machined aluminum block in whichresides a sealed crucible containing the solid material to be vaporized,entirely surrounded by a closed-circuit water bath, which is itselfenclosed by the aluminum block. The bath is held at a well-definedtemperature by a closed-loop temperature control system linked to thevaporizer. The closed-loop temperature control system incorporates a PID(Proportional Integral Differential) controller. The PID controlleraccepts a user-programmable temperature setpoint, and activates aresistive heater (which is mounted to a heater plate in contact with thewater bath) to reach and maintain it's setpoint temperature through athermocouple readback circuit which compares the setpoint and readbackvalues to determine the proper value of current to pass through theresistive heater. To ensure good temperature stability, a water-cooledheat exchanger coil is immersed in the water bath to continually removeheat from the bath, which reduces the settling time of the temperaturecontrol system. The temperature difference between the physicallyseparate heater plate and heat exchanger coil provides flow mixing ofthe water within the bath through the generation of convective currents.As an added mixing aid, a rotating magnetic mixer paddle can beincorporated into the water bath. Such a temperature control system isstable from 20C to 100C. The flow of gas from the vaporizer to theionization chamber is determined by the vaporizer temperature, such thatat higher temperatures, higher flow rates are achieved.

The flow of gas from a vaporizer to the ionization chamber is determinedby the vaporizer temperature, such that at higher temperatures, higherflow rates are achieved.

According to preferred embodiments of the invention, the vaporizercommunicates with the ionization chamber via a relativelyhigh-conductance path between the crucible and the ionization chamber.This is preferably achieved by incorporating a relatively short,large-diameter, line-of-sight conduit between the two components.High-conductance gate valves (large diameter gates with a thindimensioned housing) are used in the flow path between the vaporizer andsource body, so as not to limit this conductance. By providing a highconductance for the transport of vapor to the ionization chamber, thepressure within the vaporizer and the temperature excursion required arelower than in prior vaporizers.)

In one embodiment according to the invention a relatively lowconductance supply path is achieved employing a 5 mm diameter, 20 cmlong conduit, providing a conductance of about 7×10⁻² LUs betweencrucible and ionization chamber. This would require a pressure withinthe vaporizer of about 2 Torr to establish an ionization chamberpressure of about 4.5 mTorr. Another embodiment employs an 8 mm diameterconduit of the same length, providing a conductance of about 3×10⁻¹ L/s,allowing a pressure within the vaporizer of 0.5 Torr to achieve the sameflow rate of material, and hence the same pressure of 4.5 mTorr withinthe ionization chamber.

The static vapor pressure of a material at a given temperature and thedynamic pressure in the vaporizer crucible during the evolution andtransport of vapor out of the crucible during operation are not thesame. In general, the steady-state dynamic pressure is lower than thestatic vapor pressure, the extent depending on the distribution ofsource material within the vaporizer crucible, in addition to otherdetails of construction. According to the invention, the conductancesare made large to accommodate this effect. In addition, in certainpreferred embodiments, the added openness of the ionization chamber tothe vacuum environment of the source housing due to electron entranceand exit ports into the ionization chamber requires about twice the flowof gaseous material as a conventional Bernas-style source. Generallyaccording to the invention, it is preferred that the conductance be inthe range of about 3×10⁻² to 3×10⁻¹ L/s, preferably the length of theconduit being no less than 30 cm while its diameter is no less thanabout 5 mm, the preferred diameter range being between 5 and 10 mm.Within these limits it is possible to operate at much lower temperaturesthan conventional vaporizers, no large addition of temperature beingrequired to elevate the pressure to drive the flow to the ionizationchamber. Thus the temperature-sensitive materials are protected and abroad range of materials are enabled to be vaporized within a relativelysmall temperature range.

In several of the embodiments of the vaporizer presented, theconstruction of the vaporizer, following these guidelines, allowsoperation at temperatures between 20C and 10C or 200C. Given the highconductance of the vaporizer, and such temperature ranges, I haverealized that the wide range of solid source materials that can beaccommodated include some materials which have not previously been usedin ion implantation due to their relatively low melting point. (Itgenerally being preferred to produce vapors from material in solidform).

An additional advantage of enabling use of only a relatively lowpressure of vaporized gas within the crucible is that less material canbe required to establish the desired mass flow of gas than in priordesigns.

In another embodiment a different vaporizer PID temperature controlleris employed. In order to establish a repeatable and stable flow, thevaporizer PID temperature controller receives the output of anionization-type pressure gauge which is typically located in the sourcehousing of commercial ion implanters to monitor the sub-atmosphericpressure in the source housing. Since the pressure gauge output isproportional to the gas flow into the ion source, its output can beemployed as the controlling input to the PID temperature controller. ThePID temperature controller can subsequently raise or diminish thevaporizer temperature, to increase or decrease gas flow into the source,until the desired gauge pressure is attained.

Thus, two useful operating modes of a PID controller are defined:temperature-based, and pressure-based.

In another embodiment, these two approaches are combined such thatshort-term stability of the flow rate is accomplished by temperatureprogramming alone, while long-term stability of the flow rate isaccomplished by adjusting the vaporizer temperature to meet a pressuresetpoint. The advantage of such a combined approach is that, as thesolid material in the vaporizer crucible is consumed, the vaporizertemperature can be increased to compensate for the smaller flow ratesrealized by the reduced surface area of the material presented to thevaporizer.

In another preferred embodiment of the vaporizer, a fluid heat transfermedium is not used. Rather than a water bath, the crucible is integralwith the machined body of the vaporizer, and heating and coolingelements are embedded into the aluminum wall of the vaporizer. Theheating element is a resistive or ohmic heater, and the cooling elementis a thermoelectric (TE) cooler. The vaporizer is also encased inthermal insulation to prevent heat loss to the ambient, since thedesired vaporizer temperature is typically above room temperature. Inthis embodiment, the heating/cooling elements directly determine thetemperature of the walls of the vaporizer, and hence the temperature ofthe material within the crucible, since the material is in directcontact with the walls of the vaporizer which is e.g. machined of asingle piece of aluminum. The same PID temperature controller techniquescan be used as in the previously described embodiment, enabling thevaporizer to reach a temperature in excess of 100C, preferably up toabout 200C.

In another embodiment, the vaporizer consists of two mating, butseparate components: a vaporizer housing and a crucible. The crucible isinserted into the housing with a close mechanical fit. The surface ofthe vaporizer housing which makes contact with the crucible contains apattern of rectangular grooves, into which subatmospheric pressurizedconductive gas is introduced. The pressurized gas provides sufficientthermal conductivity between the crucible and the temperature-controlledhousing to control the temperature of the crucible surface in contactwith decaborane or other solid feed material to be vaporized. Thisembodiment allows the crucible to be easily replaced during service ofthe vaporizer. The same PID temperature controller techniques can beused as in the previously described embodiment.

In some preferred embodiments, the vaporizer, while still close to theionization chamber, communicating with it through a high conductancepath, is physically located outside of, and removably mounted to, themain mounting flange of the ion source and the vaporizer communicatesthrough the main mounting flange to the ionization chamber locatedwithin the vacuum system.

In some preferred embodiments, two vaporizers, independently detachablefrom the remainder of the ion source, are provided, enabling onevaporizer to be in use while the other, detached, is being recharged orserviced.

Vaporizer valve: In the above described vaporizer embodiments, thevapors leave the vaporizer and enter the adjacent ionization chamber ofthe ion source through an aperture, which is preferably coupled to athin, high conductance gate valve with a metal seal or other thermallyconductive seal placed between the vaporizer and ionization chamber. Thegate valve serves to separate the vaporizer from the ionization chamber,so that no vapor escapes from the vaporizer when the valve is shut, buta short, high-conductance line-of-sight path is established between theionization chamber and vaporizer when the valve is open, thus allowingthe vapors to freely enter the ionization chamber. With the valve in theclosed position, the vaporizer with the valve attached may be removedfrom the ion source without releasing the toxic vaporizer materialcontained in the crucible. The ion source may then be sealed byinstalling a blank flange in the position previously occupied by thevaporizer valve. In another embodiment, two isolation valves areprovided in series, one associated with the removable vaporizer and oneassociated with all of the other components of the ion source, with thedisconnect interface being located between the two valves. Thus bothparts of the system can be isolated from the atmosphere while the partsare detached from one another. One of the mating valves (preferably, thevalve isolating the ion source body) has a small, valved roughing portintegrated internal to the valve body, which enables the air trapped inthe dead volume between them to be evacuated by a roughing pump afterthe two valves are mated in a closed position. If the source housing ofthe implanter is under vacuum, the vaporizer can be installed with itsvalve in a closed state after being refilled. It is mated to the closedvalve mounted to the ion source in the implanter. The vaporizer valvecan then be opened and the vaporizer volume pumped out through theroughing port (along with the gas trapped in the dead volume between thevalves). Then the ion source valve can be opened, without requiringventing of the source housing. This capability greatly reduces theimplanter down time required for servicing of the vaporizer. In anothersystem, two such vaporizers, each with two isolation valves in series,as described, are provided in parallel, suitable to vaporize differentstarting materials, or to be used alternatively, so that one may beserviced and recharged while the other is functioning.

Gas feed: In order to operate with gaseous feed materials, ionimplanters typically use gas bottles which are coupled to a gasdistribution system. The gases are fed to the ion source via metal gasfeed lines which directly couple to the ion source through a sealed VCRor VCO fitting. In order to utilize these gases, embodiments of the ionsource of the present invention likewise have a gas fitting whichcouples to the interior of the ionization chamber and connects to a gasdistribution system.

Ionization chamber: The ionization chamber defines the region to whichthe neutral gas or vapor fed to the source is ionized by electronimpact.

In certain preferred embodiments, the ionization chamber is in intimatethermal and mechanical contact with the high conductance vaporizer valveor valves through thermally conductive gaskets, which are likewise inintimate thermal contact with the vaporizer through thermally conductivegaskets. This provides temperature control of the ionization chamberthrough thermal contact with the vaporizer, to avoid heat generated inthe ionization chamber from elevating the temperature of the walls ofthe chamber to temperatures which can cause decaborane or otherlow-temperature vaporized materials or gases to break down anddissociate.

In other embodiments, the ionization chamber, as a removable component,(advantageously, in certain instances, a regularly replaced consumablecomponent) is maintained in good heat transfer relationship with atemperature-controlled body, such as a temperature controlled solidmetal heat sink having a conventional water cooling medium or beingcooled by one or more thermoelectric coolers.

The ionization chamber in preferred embodiments suitable for retrofitinstallation is sized and constructed to provide an ionization volume,extraction features, and ion optical properties compatible with theproperties for which the target implanter to be retrofitted wasdesigned.

In preferred embodiments, the ionization chamber is rectangular, made ofa single piece of machined aluminum, molybdenum, graphite, siliconcarbide or other suitable thermally conductive material. Because contactof the ionization chamber with a fluid transfer medium is avoided indesigns presented here, in certain instances the ionization chamber andextraction aperture are uniquely formed of low cost graphite, which iseasily machined, or of silicon carbide, neither of which creates risk oftransition metals contamination of the implant. Likewise for the lowtemperature operations (below its melting point) an aluminumconstruction may advantageously be employed. A disposable andreplaceable ionization chamber of machined graphite or of siliconcarbide is a particular feature of the invention.

The ionization chamber in certain preferred embodiments is approximately7.5 cm tall by 5 cm wide by 5 cm deep, approximating the size and shapeof commercially accepted Bernas arc discharge ionization chambers. Thechamber wall thickness is approximately 1 cm. Thus, the ionizationchamber has the appearance of a hollow, rectangular five-sided box. Thesixth side is occupied by the exit aperture. The aperture can beelongated as are the extraction apertures of Bernas arc discharge ionsources, and located in appropriate position in relation to the ionextraction optics. The flow rate of the gas fed into the ionizationchamber is controlled to be sufficient to maintain proper feed gaspressure within the ionization chamber. For most materials, includingdecaborane, a pressure between 0.5 mTorr and 5 mTorr in the ionizationchamber will yield good ionization efficiency for the system beingdescribed. The pressure in the source housing is dependent upon thepressure in the ionization chamber. With the ionization chamber pressureat 0.5 mTorr or 5 mTorr, the ion gauge mounted in the source housing,typically used in commercial ion implanters to monitor source pressure,will read about 1×10⁻⁵ Torr and 1×10⁻⁴ Torr, respectively. The flow ratefrom the vaporizer or gas feed into the ionization chamber required tosustain this pressure is between about 1 sccm and 10 sccm (standardcubic centimeters per minute).

Electron gun: For ionizing the gases within the ionization chamber,electrons of controlled energy and generally uniform distribution areintroduced into the ionization chamber by a broad, generally collimatedbeam electron gun as shown in the illustrative figures described below.In one embodiment of the invention, a high-current electron gun ismounted adjacent one end of the ionization chamber, external to thatchamber, such that a directed stream of primary energetic electrons isinjected through an open port into the ionization chamber along the longaxis of the rectangular chamber, in a direction parallel to and adjacentthe elongated ion extraction aperture. In preferred embodiments of theinvention, the cathode of the electron gun is held at an electricpotential below the potential of the ionization chamber by a voltageequal to the desired electron energy for ionization of the molecules bythe primary electrons. Two ports, respectively in opposite walls of theionization chamber are provided to pass the electron beam, one port forentrance of the beam as mentioned above, and the second port for exit ofthe beam from the ionization chamber. After the electron beam exits theionization chamber, it is intercepted by a beam dump located justoutside of the ionization chamber the beam dump being aligned with theelectron entry point, and preferably maintained at a potential somewhatmore positive than that of the ionization chamber. The electron beam isof an energy and current that can be controllably varied over respectiveranges to accommodate the specific ionization needs of the various feedmaterials introduced into the ionization chamber, and the specific ioncurrents required by the ion implant processes of the end-user.

In particular embodiments, the electron gun is constructed to be capableof providing an electron beam energy programmable between 20 eV and 500eV.

The lowest beam energies in this energy range accommodate selectiveionization of a gas or vapor below certain ionization thresholdenergies, to limit the kinds of end-product ions produced from theneutral gas species. An example is the production of B₁₀H_(x) ⁺ ionswithout significant production of B₉H_(x) ⁺, B₈H_(x) ⁺, or otherlower-order boranes frequently contained in the decaborane crackingpattern when higher electron impact energies are used.

The higher beam energies in the energy range of the electron gun areprovided to accommodate the formation of multiply-charged ions, forexample, As⁺⁺ from AsH₃ feed gas. For the majority of ion productionfrom the various feed gases used in semiconductor manufacturing,including the production of B₁₀H_(x) ⁺ from decaborane, an electron beamenergy between 50 eV and 150 eV can yield good results.

In preferred embodiments, the electron gun is so constructed that theelectron beam current can be selected over a range of injected electronbeam currents between 0.1 mA and 500 mA, in order to determine the ioncurrent extracted from the ion source in accordance with the implantdemand. Control of electron current is accomplished by a closed-loopelectron gun controller which adjusts the electron emitter temperatureand the electron gun extraction potential to maintain the desiredelectron current setpoint. The electron emitter, or cathode, emitselectrons by thermionic emission, and so operates at elevatedtemperatures. The cathode may be directly heated (by passing an electriccurrent through the cathode material), or indirectly heated. Cathodeheating by electron bombardment from a hot filament held behind thecathode is an indirect heating technique well-practiced in the art. Thecathode may be made of tungsten, tantalum, lanthanum hexaboride (LaB₆),or other refractory conductive material. It is realized that LaB₆ offersa particular advantage, in that it emits copious currents of electronsat lower temperatures than tungsten or tantalum. As discussed furtherbelow, the preferred separate mounting of the electron beam gun,thermally isolated from the ionization chamber, is an advantageousfactor in keeping the ionization chamber cool.

Electron beam guns having cathodes mounted close to the ionizationchamber on a cooled support, which discharge directly into the chamber,are shown in the first two embodiments described below.

Further advantages are obtained in certain embodiments by use of anelongated electron gun design, i.e. typically longer that the length ofthe ionization chamber transmitted by the beam. This enables the heatedcathode of the gun to be located quite far from the ionization chamber,completely thermally isolated from it, and enables use of a small highlyefficient cathode by combination with telescopic electron optics toachieve the desired broad electron beam and desired electron densityacross the beam cross section (profile). A zoom lens can advantageouslyenable variation of the cross-section of the electron beam thattransmits the ionization chamber to match the size of the selectedaperture and beam current.

In an advantageous, space-efficient design, the elongated electron gunis mounted parallel to the direction of extraction of the ion beam, withthe cathode located near or even outside, beyond the mounting flange ofthe ion source, and associated at its other end with an electron beammirror that deflects the beam to transmit the ionization chamber.

In new implanter designs in which there are not as many predeterminedspace constraints, the described elongated electron beam gun, withrelatively small emitter surface, and associated zoom lens can bearranged in line with the direction of transmit of the electron throughthe ionization chamber, no diverting mirror being employed.

In a high current design an acceleration-deceleration system alignedwith the direction of transmit through the ionization chamber isadvantageous in a number of respects, especially when employing anaccel-decel system for maximizing the electron flow through theionization chamber.

The electron beam, however produced, has a significant cross-sectionalarea, i.e. it is a broad generally collimated beam as it transmits theionization chamber, to the beam dump with which it is aligned. Inpreferred embodiments, the electron beam within the ionization chamberhas a generally rectangular cross section, e.g. in one embodimentapproximately 13 mm×6 mm as injected into the ionization chamber, tomatch with a relatively wide extraction aperture of a high currentmachine, or the rectangular cross section is e.g. of a squarecross-section profile for use with a narrower ion extraction aperture.In the case of direct injection, the shape of the injected electron beamcan be determined by the shape of the electron optics, e.g. the grid andanode apertures of an electron gun, which, for example, may both beapproximately 13 mm×6 mm, and also by the shape of the cathode orelectron emitter, which, for the first example given, is somewhat largerthan the grid and anode apertures, approximately 15 mm×9 mm. Theadvantage of generating a generally rectangular electron beam profile isto match the conventionally desired ion beam profile as extracted fromthe ion source, which is also rectangular. The rectangular exit aperturefrom which the ion beam is extracted is approximately 50 mm tall by 3.5mm wide in many high-current implanters; in such cases the electron beam(and thus the ions produced by electron impact) can present a profile tothe exit aperture within the ionization chamber of approximately 64mm×13 mm. If the end-user wishes, an enlarged exit aperture may beemployed to obtain higher extracted currents.

As mentioned above, preferably in the walls of the ionization chamber,there are both an electron entrance port and an aligned electron exitport for the electron beam, which departs from the conventionallyemployed Bernas ion source. In Bernas ion sources, energetic electronsproduced by an emitter. located typically internal to the ionizationchamber. strike the walls of the chamber to form the basis of an “arcdischarge”. This provides a substantial heat load which elevates thetemperature of the ionization chamber walls. In the present invention,the ionizing electrons (i.e the energetic or “primary” electrons) passthrough the ionization chamber to the defined beam dump, substantiallywithout intercepting the general chamber walls. “Secondary” electrons,i.e. low-energy electrons produced by ionization of the feed gas, stillcan reach the general walls of the ionization chamber but since theseare low energy electrons, they do not provide significant heat load tothe walls. The feature of through-transmit of the primary electronsallows the ionization chamber to be conductively cooled, e.g. by thevaporizer, or by a cooled block against which the ionization chamber ismounted in substantial thermal contact, without providing a large heatload on the temperature controller of the vaporizer or block. To avoidthe heat generated by the electron gun and the energetic electron beam,the electron gun and the electron beam dump are mounted in thermallyisolated fashion, preferably either or both being mounted on respectivewater-cooled parts of a cooled mounting frame. This frame is dynamicallycooled, e.g. by high-resistivity, de-ionized water commonly available incommercial ion implanters.

Cooled mounting frame and Beam Dump: The cooled mounting frame is e.g. awater-cooled sheet metal assembly on which the electron gun and theelectron beam dump may be mounted. The frame consists of two separatemechanical parts which allow the electron gun and the beam dump to beindependently biased. By mounting these two components to this frame, aheat load to the ionization chamber can be substantially avoided. Theframe provides a mechanical framework for the thus-mounted components,and in addition the frame and the mounted components can be held at anelectric potential different from the potential of the ionizationchamber and vaporizer by mounting to the ion source assembly onelectrically insulating standoffs.

In embodiments discussed below, the beam dump is discretely defined andisolated, preferably being removed from direct contact with theionization chamber, with the electron beam passing through an exit portin the ionization chamber prior to being intercepted by the beam dump.The beam dump can readily be maintained at a potential more positivethan the walls of the chamber to retain any secondary electrons releasedupon impact of primary electrons up on the beam dump. Also, the beamdump current can be detected for use in the control system as well asfor diagnostics. Also, in a multi-mode ion source, by being electricallyisolated, the voltage on the dump structure can be selectively changedto negative to serve an electron-repeller (anticathode) function, asdescribed below.

In another construction, the distinctly defined beam dump though can bein physical contact with the exit port in such a way that thermalconduction between the cooled beam dump and the exit port is poor e.g.,by point contact of discrete elements. Electrical insulation, which hasthermal insulation properties as well, can be provided to enable avoltage differential to be maintained while preventing heating of thegeneral walls of the ionization chamber. One advantage of thisembodiment is a reduced conduction of the source gas out of theionization chamber, reducing gas usage.

The extraction of ions from the ionization chamber is facilitated by anasymmetric relationship of the electron beam axis relative to thecentral chamber axis, locating the site of ionization closer to theextraction aperture. By maintaining a voltage on the aperture platethrough which the ions are extracted that is lower than that of theother chamber walls, the ions are drawn toward the extraction path.

In use of the ion source in a mode different from that used fordecaborane as described above, e.g. using BF₃ feed gas, the electronbeam dump may be biased to a negative potential relative to theionization chamber, e.g. to a voltage approximating that of the cathodepotential, in a “reflex geometry” whereby the primary electrons emittedby the electron gun are reflected back into the ionization chamber andto the cathode, and back again repeatedly, i.e. instead of serving as abeam dump, in this mode the dump structure serves as a “repeller”, oranticathode. An axial magnetic field may also be established along thedirection of the electron beam by a pair of magnet coils external to theion source, to provide confinement of the primary electron beam as it isreflected back and forth between the cathode and beam dump. This featurealso provides some confinement for the ions, which may increase theefficiency of creating certain desired ion products, for example B⁺ fromBF₃ feed gas. Such a reflex mode of operation is known per se by thosepracticed in the art, but is achieved here in a unique multi-mode ionsource design capable of efficiently producing e.g. decaborane ions.

A novel multimode ion source includes an electron gun for the purposesas described, disposed coaxially within a magnet coil that is associatedwith the source housing and ionization chamber contained within.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic view of a prior art ion implanter;

FIG. 2 is a diagrammatic cross-sectional view of a Bernas arc dischargeion source, illustrative of the ion source for which the implanter ofFIG. 1 was designed;

FIG. 3 is a longitudinal cross-sectional view of an embodiment of theion source of the present invention with associated vaporizer;

FIG. 3A is a cross-sectional view, similar to a part of FIG. 3, showinganother embodiment of a vaporizer;

FIG. 3B illustrates the removable feature of the vaporizer of FIG. 3A,using a conventional mounting flange while FIG. 3C illustrates detachingthe vaporizer and valve from the ion source;

FIG. 3D illustrates a two-valve embodiment in which separation of thevaporizer from the ion source can occur between the two valves;

FIG. 3E illustrates a dual vaporizer embodiment;

FIG. 3F shows another embodiment of a vaporizer similar to FIG. 3A, butwith a separate crucible and with gas-mediated conduction betweenvaporizer housing and crucible, and between a heat exchanger and thehousing.

FIG. 4 is a side cross-sectional view taken on line 4-4 of FIG. 3 whileFIG. 4A is a top view taken on line 4A-4A of FIG. 4;

FIGS. 4B and 4C are views similar to that of FIG. 4, of otherarrangements of the discretely defined electric beam dump;

FIGS. 4D and 4E, side and top views similar respectively to FIGS. 4 and4A, show a conductively cooled ionization chamber assembly having adisposable inner ionization chamber.

FIG. 4F is a three dimensional representation of a broad, collimatedelectron beam and its relation to the ion extraction aperture of theembodiment of FIGS. 3 and 4;

FIG. 5 is a view similar to FIG. 4F of the relationship of a broadelectron beam and ion extraction aperture of narrower dimension;

FIG. 6 is a front view of the aperture plate of the ion source of FIG.3;

FIG. 7 is an illustration of an indirectly heated cathode arrangement;

FIG. 8 illustrates the ion source of FIGS. 3-6 installed in a retrofitvolume of a pre-existing ion implanter while FIG. 8A illustrates, on asmaller scale, the entire implanter of FIG. 8;

FIG. 9, similar to FIG. 8, shows an ion source employing an elongatedright angle electron gun and an angled mirror while FIG. 9A illustratesthe entire implanter into which the embodiment of FIG. 9 is retrofit;

FIG. 9B is a view similar to a portion of FIG. 9 on an enlarged scale,illustrating a demountable ionizing chamber directly mounted upon awater-cooled block;

FIG. 10 is a side view on an enlarged scale of a preferred embodiment ofthe elongated electron gun of FIG. 9;

FIG. 11 is an enlarged diagram of the extraction stage of the gun ofFIG. 10;

FIG. 12 illustrates the trajectories of electrons through the extractionstage of FIG. 11;

FIG. 13 is a diagrammatic view of a 5-element zoom lens;

FIGS. 13A through 13D illustrate various operating modes of the lenssystem of FIG. 13;

FIG. 14 is a plot of the zoom voltage line;

FIG. 15 is a diagram of the operator interface of a conventional Bernasarc discharge ion source while FIG. 15A is a similar view of a Bernassource with indirectly heated cathode;

FIG. 16 is a view of a Bernas operator interface combined with a novelconfigurable universal controller that controls a broad E-Beam ionsource according to the invention;

FIG. 16A is a view similar to FIG. 16 of the control system for anelongated E-Beam embodiment of the invention;

FIG. 16B is a diagram of a preferred embodiment of a temperature controlsystem for the vaporizer of FIGS. 3 and 3A;

FIG. 17 is a diagrammatic illustration of a semiconductor device,illustrating standard CMOS ion implantation applications.

FIG. 18 is a diagram of a high-current electron gun incorporated into apreferred embodiment of the ion source, where the optical axis of theelectron gun is parallel to the long axis of the ionization chamber,showing the approximate scale and operating voltages of the differentelements;

FIG. 18A shows the electron optics of the ion source of FIG. 18, wherethe focusing properties of a double-aperture lens are illustrated byobject and image points, and also the detailed mechanical structure ofthe ionization chamber and beam dump are illustrated;

FIG. 18B illustrates mounting the ion source of FIGS. 18 and 18A into anexisting ion implanter, and a special arrangement of the electron gunand magnet coils.

FIG. 19 is a top view of an aperture plate that has provisions forreceiving a bias voltage relative to the voltage of the remaining wallsof an ionization chamber, while FIGS. 19A and 19B, taken on respectivelines in FIG. 19, are side views respectively of the inside face of theaperture plate, facing the interior of the ionization chamber and theoutside face, directed toward the extraction optics.

FIG. 19C is an edge view of an aperture plate illustrating it's mountingto the main body of the ionization chamber by insulating stand offs.

FIGS. 20A and 20B an side views of the inside face and outside face ofan aperture insert plate of another embodiment while FIG. 20C is a sideview of an insulator frame into which the insert plate of FIGS. 20A and20B may be mounted.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 shows in schematic an embodiment of ion source 1. The vaporizer 2is attached to the vaporizer valve 3 through an annular thermallyconductive gasket 4. The vaporizer valve 3 is likewise attached to themounting flange 7, and the mounting flange 7 is attached to ionizationchamber body 5 by further annular thermally conductive gaskets 6 and 6A.This ensures good thermal conduction between the vaporizer, vaporizervalve, and ionization chamber body 5 through intimate contact viathermally conductive elements. The mounting flange 7 attached to theionization chamber 5, e.g., allows mounting of the ion source 1 to thevacuum housing of an ion implanter, (see FIG. 8) and contains electricalfeedthroughs (not shown) to power the ion source, and water-coolingfeedthroughs 8, 9 for cooling. In this preferred embodiment, waterfeedthroughs 8, 9 circulate water through the cooled mounting frame 10to cool the mounting frame 10 which in turn cools the attachedcomponents, the electron beam dump 11 and electron gun 12. The exitaperture plate 13 is mounted to the face of the ionization chamber body5 by metal screws (not shown). Thermal conduction of the ion exitaperture plate 13 to the ionization chamber body 5 is aided byconductive annular seal 14 of metal or a thermally conductive polymer.

When the vaporizer valve 3 is in the open position, vaporized gases fromthe vaporizer 2 can flow through the vaporizer valve 3 to inlet channel15 into the open volume of the ionization chamber 16. These gases areionized by interaction with the electron beam transported from theelectron gun 12 to the electron beam dump 11. The ions produced in theopen volume can then exit the ion source from the exit aperture 37,where they are collected and transported by the ion optics of the ionimplanter.

The body of vaporizer 2 is made of machined aluminum, and houses a waterbath 17 which surrounds a crucible 18 containing a solid feed materialsuch as decaborane 19. The water bath 17 is heated by a resistive heaterplate 20 and cooled by a heat exchanger coil 21 to keep the water bathat the desired temperature. The heat exchanger coil 21 is cooled byde-ionized water provided by water inlet 22 and water outlet 23. Thetemperature difference between the heating and cooling elements providesconvective mixing of the water, and a magnetic paddle stirrer 24continuously stirs the water bath 17 while the vaporizer is inoperation. A thermocouple 25 continually monitors the temperature of thecrucible 18 to provide temperature readback for a PID vaporizertemperature controller (not shown). The ionization chamber body 5 ismade of aluminum, graphite, silicon carbide, or molybdenum, and operatesnear the temperature of the vaporizer 2 through thermal conduction. Inaddition to low-temperature vaporized solids, the ion source can receivegases through gas feed 26, which feeds directly into the open volume ofthe ionization chamber 16 by an inlet channel 27. Feed gases providedthrough channel 27 for the ion implantation of semiconductors includeAsH₃, PH₃, SbF₅, BF₃, CO₂, Ar, N₂, SiF₄, and GeF₄, and with importantadvantages GeH₄, SiH₄, and B₂H₆, described below. When the gas feed 26is used to input feed gases, the vaporizer valve 3 is closed. In thecase of a number of these gases, the broad beam electron ionization ofthe present invention produces a mid-to-low ion current, useful formid-to-low dose implantations. For higher doses, an embodiment capableof switching mode to a reflex geometry, with magnetic field, can beemployed.

The vaporizer 2 of FIG. 3, or that of FIG. 3A to be described, can bedemounted from the ion source 1 by closing the vaporizer valve 3 andremoving the unit at seal 6, (parting line D), compare FIGS. 3B and 3C.This is useful for recharging the solid feed material in the crucible18, and for maintenance activities.

In the embodiment of FIG. 3D, two valves, 3 and 3A are provided inseries, valve 3 being permanently associated, as before, with removablevaporizer 28 and valve 3A being permanently associated with mountingflange 7, with the demounting plane D disposed between the two valves.

In the embodiment of the ion source shown in FIG. 3A, the vaporizer 28is of a different design from that of FIG. 3, while the rest of the ionsource is the same as in FIG. 3. In vaporizer 28, there is no water bathor water-fed heat exchanger. Instead, the volume occupied by water bath17 in FIG. 3 is occupied by the machined aluminum body 29 of vaporizer28. A resistive heater plate 20 is in direct thermal contact with thevaporizer body 29 to conductively heat the body 29, and a thermoelectric(TE) cooler 30 is in direct thermal contact with the vaporizer body 29to provide conductive cooling. A thermally insulating sleeve 31surrounds the vaporizer 28 to thermally insulate the vaporizer fromambient temperature. If desired, several heater plates 20 and TE coolers30 can be distributed within the vaporizer body 29 to provide moreconductive heating and cooling power, and also to provide a morespatially uniform temperature to the crucible. This construction permitsthe vaporizer to operate at temperatures in excess of 100C, up to about200C.

FIG. 3B illustrates an embodiment in which successive mounting flangesof the series of vaporizer 28, isolation valve 3 and the ion source 1,are of increasing size, enabling access to each flange for detachment.Mounting flange 70 enables bolt-on of the assembled ion source to theion source housing, see e.g. FIG. 8. Mounting Flange 7 a enablesattachment and detachment of the vaporizer 28 and its associated valve 3from flange 7 at parting line D, see FIG. 3C. Mounting Flange 7 benables detachment of the valve 3 from the main body of the vaporizerfor maintenance or recharging the vaporizer.

The embodiment of FIG. 3D has two valves 3 and 3 a, valve 3 normallystaying attached to the vaporizer and valve 3 a normally attached to ionsource mounting flange 7. These enable isolation of both the vaporizer28 and the ion source 1 before demounting the vaporizer at parting lineD. The body of mated valve 3 a includes roughing passage 90 connected byvalve 92 to roughing conduit 91 by which the space between the valvesmay be evacuated, and, upon opening valve 3, by which the vaporizer maybe evacuated prior to opening valve 3 a. Thus attachment of vaporizer 28need not adversely affect the vacuum being maintained in the ion sourceand beam line.

The vent line 93, and associated valve 94 enables relief of vacuumwithin the vaporizer prior to performing maintenance and as well may beused to evacuate and outgas the vaporizer after recharging, to conditionit for use.

The embodiment of FIG. 3E illustrates a dual vaporizer construction,having the capabilities previously described. The vapor passage 15 inmetal block heat sink 5 a bifurcates near mounting flange 7, thebranches 15′ leading to respective demountable vaporizers VAP 1 and VAP2, each having two isolation valves separable at parting line D. As morefully described with respect to FIG. 9B, the ionization chamber body 5 bis of discrete construction, demountably mounted in intimate heattransfer relationship to temperature controlled mounting block 5 a.

Separate coolant passage 66 and 67 telescopically receive so-calledsquirt tubes which centrally conduct cold, deionized water to the deadend of the passage. The emerging cooled water has its maximum effectthat that point, in the outward regions of respectively the mountingblock 5 a and the cooled frame 10, the water returns through the annularspace defined between the exterior of the squirt tube and the passage inwhich the tube resides.

FIG. 3F shows a vaporizer similar to that of FIG. 3A, but instead of aone-piece aluminum construction, the body of the vaporizer has twomating, but separate components: a vaporizer housing 29 ¹ and a crucible18 ¹. The crucible is inserted into the housing 29 ¹ with a closemechanical fit. The surface of the vaporizer housing which makes contactwith the crucible contains a pattern of rectangular grooves, into whichpressurized gas (typically at subatmospheric pressure) is introducedthrough gas inlet 93 ¹. The presurized gas provides sufficient thermalconductivity between the crucible 18 ¹ and the temperature-controlledhousing 291 to control the temperature of the crucible surface 65 incontact with decaborane or other solid feed material 19 to be vaporized.This embodiment allows the crucible 181 to be easily replaced duringservice of the vaporizer. Gas is also fed into the volume surroundingheat exchanger 21, to promote thermal conduction between the heatexchanger 21 and the housing 29 ¹. The heat exchanger 21 is shown as awater-fed coil, but may alternatively comprise a TE cooler, such ascooler 30 in FIG. 3A.

Referring to FIG. 4, in operation of the ion source 1, an electron beam32 is emitted from the cathode 33 and focused by the electron optics 34to form a broad, collimated beam, consisting of dispersed electrons(preferably generally uniformly dispersed). The electron beam is widerperpendicular to the ion beam axis than it is along that axis. FIG. 4illustrates the geometry of the ion source with the exit aperture plate13 removed; the ion beam axis points out of the plane of the paper, seeFIG. 4A. The distribution of ions created by neutral gas interactionwith the electron beam corresponds generally to the defined profile ofthe electron beam.

The electron beam passes through a rectangular entrance port 35 in theionization chamber and interacts with the neutral gas within the openvolume 16, defined within the ionization chamber body 5. The beam thenpasses directly through a rectangular exit port 36 in the ionizationchamber and is intercepted by the beam dump 11, which is mounted on thewater-cooled mounting frame 10. Beam dump 11 is maintained at a positivepotential relative to the electron gun, and preferably slightly positiverelative to the walls of the ionization chamber as well. Since the heatload generated by the hot cathode 33 and the heat load generated byimpact of the electron beam 32 with the beam dump 11 is substantial,their location outside of the ionization chamber open volume 16 preventstheir causing dissociation of the neutral gas molecules and ions. Theonly heat load from these elements to the ionization chamber is limitedto modest radiation, so the ionization chamber can be effectively cooledby thermal conduction to the vaporizer 2 (FIG. 3) or by conduction to amassive mounting block 5 a (FIGS. 3E, 9B). Thus, the general walls ofthe ionization chamber can be reliably maintained at a temperature belowthe dissociation temperature of the neutral gas molecules and ions. Fordecaborane, this dissociation temperature is about 350C. Since the ionexit aperture 37 in plate 13, shown in FIGS. 4B, 5 and 6, is a generallyrectangular aperture, the distribution of ions created adjacent to theaperture by the broad, collimated beam of generally uniformly dispersedelectrons should be likewise uniform. In the ionization of decaboraneand other large molecules, according to this embodiment, an arc plasmais not sustained, but rather the gas is ionized by directelectron-impact ionization by the primary (energetic) electrons, in theabsence of containment by any major confining magnetic field. Theabsence of such magnetic field limits the charge-exchange interactionsbetween the ions and relatively cool secondary electrons as they are notstrongly confined as they are in an arc plasma (confined secondaryelectrons can cause loss of the ions of interest through multipleionizations). The decaborane ions are generated in the widelydistributed electron beam path. This reduces the local ion densityrelative to other conventional ion sources known in the art.

The absence of magnetic field can improve the emittance of the extractedion beam, particularly at low (e.g., 5 keV) extraction energy. Theabsence of an arc plasma as in a Bernas source also can improveemittance since there is no plasma potential present in the ionizationand extraction region. (I recognize that the presence of an arc plasmapotential in conventional plasma-based ion sources introduces asignificant random energy component to the ions prior to beingextracted, which translates directly into an added angular spread in theextracted ion beam. The maximum angular spread θ due to a plasmapotential φ is given by: θ=√2 arcsin {φ/E}^(1/2), where E is the beamenergy. For example, for a plasma potential of 5 eV and a beam energy of5 keV, θ=2.5 deg. In contrast, the random energy of ions produced bydirect electron-impact ionization is generally thermal, much less than 1eV.)

FIG. 4A shows a top view of the electron exit port 36 in the open volume16 of ionization chamber body 5, and its proximity to the ion exitaperture 37 in aperture plate 13. To enable the ions to be removed fromthe ionization chamber by penetration of an electrostatic extractionfield outside of the ion source 1 through the ion exit aperture 37, theelectron beam 32 and electron exit port 36 are situated close to theexit aperture plate 13 and its aperture 37. For example, a separation ofbetween 6 mm and 9 mm between the edge of the ionization region and theion extraction aperture can result in good ion extraction efficiency,the efficiency improving with larger width extraction apertures.Depending upon the particular parameters chosen, the broad, collimatedelectron beam 32 may not fully retain its rectangular profile due toscattering, and also due to space charge forces within the electron beam32. The electron exit port 36 is sized appropriately in accordance withsuch design choices to allow passage of the electron beam withoutsignificant interception by the general walls of the ionization chamberbody 5. Thus, in certain advantageous instances, port 36 is larger thanport 35 so that it is aligned to receive and pass at least most of theresidual electron beam.

The embodiment of FIG. 4B illustrates a discretely defined beam dump 11′which is sized and shaped to fit within port 36′ such that its inner,electron receiving surface lies flush with the inner surface of thesurrounding end wall of the chamber body 5. Beam dump 11′ is mountedupon and is cooled by cooled frame 10, as before. As shown, a clearancespace c, e.g., of 1 mm, is maintained between the beam dump structureand the wall of the chamber. Preferably, as shown, the structures arecooperatively shaped as in a labyrinth L, to limit the outflow of thedopant gas or vapor, while maintaining thermal and electrical isolationof the dump structure 11′ from the walls of the ionization chamber,maintaining electrical isolation of the beam dump 11′ while preventingloss of dopant gas or vapor.

In the embodiment of FIG. 4C electrical insulation Z fills the spacebetween the beam dump and the wall of the ionization chamber,maintaining electrical isolation of the beam dump 11′ while preventingloss of dopant gas or vapor.

Referring to FIGS. 4D and 4E, a thermoelectrically or water-cooled outerhousing H, defines a space into which a chamber-defining member 5 c ofheat-conductive and electrically-conductive material is removablyinserted with close operational fit. Gas inlets G_(i) introduceconductive gas of a subatmospheric pressure (e.g., between 0.5 and 5Torr), that is significantly higher than that of the operational vacuumV_(c) within the overall ion source housing 49 which contains theionization chamber assembly. The conductive gas (for example, N₂, Ar, orHe) is introduced to the interface If between matching surfaces of thehousing and the chamber in regions remote from exposure of the interfaceto operational vacuum V_(o), and isolated from the vaporizer and processgas feed lines. In a preferred embodiment, the cooling gas is fedthrough an aluminum block or cooled housing and exits between thedemountable ionization chamber and the block or housing, at theinterface between them, into cooling channels machined into the aluminumblock. The cooling channels have the form of linear grooves (e.g., 1 mmwide by 0.1 mm deep) which populate a significant percentage of thesurface area between the two mating components. This construction allowsthe flat mating surfaces (the grooved aluminum surface and the flatsurface of the separate ionization chamber) of the two components tomate flush with one another. Simple elastomeric o-rings encompass thesurface area which contains the cooling channel grooves, ensuring thatthe gas confined to the cooling channels is isolated from regions whichcontain feedthroughs and passages for process gas or vapor within thisinterface, and also isolates the cooling gas from the ionization volumeand from the vacuum housing. The spacing between those surfaces and thepressure of the conductive gas in the interface are so related that themean-free path of the conductive gas molecules is of the order of orless than the spacing of opposed surface portions at the interface.

The conductive gas molecules, by thermal motion, conduct heat across theinterface from the chamber wall to the surrounding cooled housingelements. Any regions of actual physical contact between the solidmaterial of the chamber body and of an outer housing element likewisepromotes cooling by conduction. It is to be noted that the mode ofconductive gas cooling described here does not depend upon convectionalgas flow, but only upon the presence in the interface of the gasmolecules. Therefore, in some embodiments, it may be preferred to formseals at the interface to capture the gas, as discussed above, althoughin other embodiments exposure of the interface at edges of the assemblywith leakage to the operational vacuum V_(o) can be tolerated just as isthe case with respect to cooling of semiconductor wafers as described,e.g., in the King U.S. Pat. No. 4,261,762.

In other embodiments, the cooling housing of the ionization chamberassembly or similar side wall elements of other structures of the ionsource are water-cooled in the manner of cooling the mounting frame 10as described herein. In some embodiments, depending upon the heat loadon the ionization chamber, the heat conduction resulting from theinclusion of thermally conductive gasket seals, as well as regions ofphysical point contact between the matching surfaces of the chamber andhousing elements is sufficient to keep the chamber within the desiredtemperature range, and the conductive gas-cooling feature described isnot employed.

It is recognized that the heat-transfer relationships described herehave general applicability throughout the ion source and the otherstructural components of the implanter as well. Thus, the temperature ofthe vaporizer may be controlled by the heat transfer from a disposablecrucible to surrounding elements via gas conduction at an interface, foroperating conditions which require less than, for example, 2 W/cm² ofheat transfer through the gas interface. Likewise, surfaces of theelectron gun, the electron beam dump, the mounting frame and theaperture plate may serve as conductors via a conductive gas interface totemperature-control elements such as the thermoelectrically orwater-cooled housing that has been described, as illustrated in FIG. 4E.

FIGS. 4F and 5 show different sizes of a broad, collimated electron beampassing through the ionization chamber, the profiles of these beamsmatched in profile to the wide and narrower apertures of the respectiveionization chambers of FIGS. 4F and 5.

FIG. 6 shows the ion exit aperture plate 13 with the axis of the ionbeam directed normal to the plane of the paper. The dimensions of theexit aperture plate conform to the dimensions of the ionization chamberwithin body 5, approximately 7.6 cm tall×5.1 cm wide. The exit apertureplate contains an opening 37 which is approximately 5.1 cm in height, s,by 1.3 cm wide, r, suitable for high current implanters, and has a bevel38 to reduce strong electric fields at its edges. It is matched by abroad, collimated electron beam having width g of 19 mm and depth p of 6mm, cross-sectional area of 114 square mm. The aperture of theembodiment of FIG. 5, has similar features but a much narrower width,e.g. a width r¹, 4 mm, matched by an electron beam of width g¹ 6 mm anda depth p¹ of 6 mm.

FIG. 7 shows the shape of the cathode 33, or electron emitter. In apreferred embodiment, it defines a planar emitting surface, it'sdimensions being roughly 15 mm long×9 mm×3 mm thick. It can be directlyheated by passing an electric current through it, or it can beindirectly heated, as shown, with an electric current flowing throughfilament 39 via leads 40, heating it to emit thermionic electrons 41. Bybiasing the filament 39 to a voltage several hundred volts below thepotential of cathode 33, thermionic electrons 41 heat the cathode 33 byenergetic electron bombardment, as is known in the art.

FIG. 8 illustrates the assembly of an ion source according to FIG. 3Ainto a retrofit volume 60 of a previously installed ion implanter whileFIG. 8A illustrates the complete ion implanter.

In this particular embodiment nothing has been disturbed except that theBernas ion source for which the implanter was originally designed hasbeen removed and, into the vacated volume 60, the ion source of FIG. 3Ahas been installed, with its flange 7 bolted to the ion source housingflange. The extraction electrodes 53 remain in their original position,and the new ion source presents its aperture 37 in the same region asdid the arc discharge Bernas source. The magnet coils 54 are shownremaining, available e.g., for operation in reflex mode if desired, orfor applying a containment field for electrons proceeding to the beamdump 11.

As shown in FIG. 9 the usual gas connections are made enabling dopantgases from sources 1, 2, 3, and 4 in the supply rack 76 of the gas box70 to be connected via inlet conduit 74 and exhausted via conduit 72 tohigh vacuum system 78.

Long E-Beam Gun Retrofit Embodiment

Referring to FIG. 9, an extended E-Beam gun is uniquely associated withan ionization chamber. The gun has zoom optics, and comprises thefollowing components: extended housing 79, feedthroughs 80, mountingflanges 81 and 81′, cathode 82, extraction stage 83, collimation lens84, zoom lens 85, and turning stage 87 comprising a 90 degree mirror.

The long gun housing 79 lies along an axis A′ parallel to the directionA of emission of the ion beam from the ion source, and within theretrofit space 60 of the previously installed implanter ion source. Thehousing extends from the feedthrough terminals 80, resident outside ofthe mounting flange 7 of the ion source, past a vacuum pump 58,terminating at mounting flange 81′ and the main ion source mountingflange 7. The electron beam optics continue alongside the ion sourceblock 5 to a point in registry with the electron inlet port 35 of theionization volume 16.

The feedthroughs comprise appropriate fittings for the power and controllines for the cathode and other stages of the gun, and cooling waterinlet and outlet for the housing, which is cooled, at least in thevicinity of the cathode. In an alternate embodiment, special cooling ofthe gun housing is not employed, the remoteness of the cathode, as shownin FIG. 9, ensuring that the ionization chamber 5 s not heated by thecathode, and any necessary cooling for protection of the vaporizer oroperating personnel being achieved by conduction to water cooledmounting flanges or the like.

With significant cost and size efficiencies, the cathode 82 is ofrelatively small size in comparison to the profile dimension of thelargest broad, aligned electron beam that is to transmit the ionizationvolume 16. It is preferably a resistance-heated or indirectly heated,planar cathode emitter plate (such as plate 33 described above inconnection with FIG. 7), made of lanthanum hexaboride (LaB₆) or ofrefractory metal such as tantalum or tungsten, to emit a generallyuniform stream of electrons to the high voltage electron extractionstage.

As shown in FIG. 9A the ion source of FIG. 9 is retrofit into vacatedvolume 60 of a previously installed ion implanter. The compact natureand arrangement of the ion source locates the prime heat source, thecathode, remotely from the ionization chamber 16 such that its heat doesnot contribute to disassociation of the fragile dopant molecules. In thecase of FIGS. 9A and 9B, heat from the ionization chamber is conductedto the vaporizer and is controlled by its temperature control.

During operation, the vacuum pump 58 in the region of the cathode 82intercepts back-streaming gas which has escaped from the ionizationchamber 16 via the electron inlet port 35. This has the importantadvantage of protecting the remote cathode 82 from contamination, andenables a very extended cathode life, a feature which is especiallyimportant to enable use of the preferred LaB₆ cathodes, which areparticularly sensitive to degradation from chemically active species.

Water Cooled Block and Demountable Ionization Chamber

In the embodiment of FIG. 9B (see also FIG. 3E) the ionization volume16′ is defined by a demountable end module Sb which is mounted withconductive thermal contact on the end of solid mounting block 5 a viathermally conductive seal 6″.

For achieving demountability, the conductive seal 6″ is compressed viametal screws through mating surfaces of the block 5 a and thedemountable end module 5 b. This construction enables the member 5 bdefining the ionization chamber 16′ to be removed from the block 5 a andreplaced with an unused member, advantageously of disposableconstruction. It also enables a different, and in some cases moreefficient cooling of walls of the ionization chamber 16′ than inprevious embodiments. For construction of the demountable member, inaddition to aluminum (which is inexpensive and less injurious to thewafers being implanted than molybdenum, tungsten or other metals iftransported to the wafer in the ion beam), the ionization chamber member5 b and exit aperture plate 13 are advantageously constructed fromgraphite or SiC, which removes altogether the possibility of metalscontamination of the wafer due to propagation from the ion source. Inaddition, demountable ionization chambers of graphite and SiC may beformed cheaply, and thus can be discarded during maintenance, being lessexpensive to be replaced than a one-piece structure.

In another embodiment, for conductively controlling the temperature ofthe block 5 a and the chamber body 5 b, they have mating smoothsurfaces, the surface of the block containing machined cooling channelswhich admit conductive cooling gas between the block 5 a and the chamberbody 5 b, so that that gas, introduced under vacuum, transfers heat byheat conduction (not convection) in accordance with the abovedescription of FIGS. 4D and 4E, and cooling techniques used for thedifferent situation of cooling wafers that are being implanted, see KingU.S. Pat. No. 4,261,762. In this case, gaskets at the vapor and gaspassages prevent mixing of the conductive heat transfer gas, such asargon, with the gas or vapor to be ionized.

As shown, block 5 a is cooled by water passages 24 a, either associatedwith its own thermal control system, FIG. 3E, or, as shown, in FIG. 9B,associated with the cooling system 24 that cools frame 10 on which thebeam dump 11 is mounted. By being based upon heat conduction throughsolid members, water contact with the walls of the ionization chamber isavoided, making it uniquely possible to fabricate the ionization chamberof materials, such as low cost machined or molded graphite, which cannotconveniently be exposed to water. The remote location of the cathode andits heat effects combine with these mounting features to achieve desiredcool-running of the ionization chamber.

Advantageous E-Beam Gun Features

Features of particularly preferred embodiments of the long E-Beam areshown in FIG. 10, with the extraction stage 83 shown in greater detailin FIG. 11. The extraction stage 83 is of cylindrical geometry, andcomprises a cathode 82, a field shaping grid electrode 100, Wehneldtelectrode 101, cylinder lens 102, and anode 103. Relative to the cathodepotential V_(c), the grid potential V_(g) is held, for example, at−2V<V_(g)<+4V and the anode potential V1 is maintained at between about200 and 1000 volts positive, depending on the desired electron energy atthe exit of the extraction stage. The Wehneldt and cylinder potentials,V_(w) and V_(s), respectively, are tuned so as to produce electrontrajectories through the extraction stage which limit filling of itslenses, and limit the beam angle of the electron trajectories at theoutput of the extraction stage. In essence, the purpose of theextraction stage is to collect the thermionically emitted electrons fromthe directly heated cathode or from the emitter surface of an indirectlyheated cathode, to provide a beam of significantly energized electronsin a beam with a desired regular profile, with a degree of uniformity ofelectron distribution and collimation that presents a good qualityobject for the downstream telescopic lens system shown in FIG. 10. Suchtuning is shown in FIG. 12 for an extraction stage which was originallydeveloped for low-energy positrons (see I. J. Rosenberg, A. H. Weiss,and K. F. Canter, Physical Review Letters 44, p. 1139, 1980). It ismodified and used for forming a broad electron beam as part of thepresent invention. The original extraction stage described by Rosenberget al. was essentially a 100% positron transmission stage designed foran extended, 10 mm diameter positron emitter. In the present E-Beam gun,the extraction stage is scaled smaller, e.g. by a factor of 0.5 toaccommodate a 5 mm diameter cathode electron emitter with the aperturediameter of grid electrode 100 5 mm and the sign of the electrodepotentials reversed to make the structure suitable for extraction ofelectrons. With this scale factor, the electron extraction stage isapproximately 27 mm long, with the cylinder lens diameter being 17.5 mm.

In FIG. 11, typical dimensions may be:

d=5 mm l₁=1.3 mm

d₁=9.5 mm l₂=2.3 mm

d₂=17.5 mm l₃=4.8 mm

d₃=9.5 mm l₄=18 mm

Where V_(c)=any range between −20 to −300 or −500 V, relative to V_(ch),the potential of the ionization chamber. Relative to V_(c), then, theother voltage values for instance, may respectively range between:−2V<V_(g)<4V0V<V_(w)<500V50V<V_(s)<500V200V<V1<1000V

Other embodiments of the electron extraction stage are possible. In oneembodiment, the emitting surface of the cathode 82 is moved forward tolie in the same plane as the grid 100, field shaping provided by thegrid aperture not being employed. In this case, grid 100 is held at thesame potential as cathode 82. Another advantageous embodiment of theextraction stage incorporates a Pierce geometry, in which the gridaperture is coplanar with the cathode, but the shape of the grid isconical, with sides inclined at an angle of 22.5°, corresponding to acone angle of 135° (see J. R. Pierce, Theory and Design of ElectronBeams, 2^(nd) edition, Van Nostrand, New York, 1954). This electrodeshaping advantageously counteracts the effects of electron space chargein the highly populated vicinity of the cathode.

In the presently preferred embodiment, the 5 mm-diameter, circularthermionic cathode plate is heated to emit an average electron currentdensity of about 200 mA/cm² from its face having an emitting area of 0.2cm², yielding 40 mA of electron current into the extraction stage. Theextraction stage serves as an injection stage for the following lenssystem which comprises collimating lens 84 followed by zoom lens 85. Inthe preferred embodiment, these lenses comprise 17.5-mm-diameter (“D”),thin-walled metal cylinders, separated by gaps equal to 0.10 D. Whendiffering potentials are applied to the thus separated cylinders, strongfocusing fields are generated at the gaps, producing lensing effects.

Referring to FIG. 10, the collimating lens 84 is an asymmetric einzellens, that is, it consists of three coaxial cylinders of length 2D, 1D,and 2D at voltages V1, V12, and V3. V1 is not equal to V3 (hence theEinzel lens is “asymmetric”). In general, the three elements (triplet)of each einzel lens acts as a single “thick” lens. In the case ofcollimating einzel lens 84, V2>V1, and lens 84 acts as an acceleratinglens. V12 is varied to adjust the focal length, hence the magnificationof the triplet. Lens 84 also acts to limit overfilling of the cylindersby the electron beam, which can produce aberrations and beam loss. Asdescribed, collimating lens 84 presents an object to the downstream zoomlens 85 with appropriate beam characteristics to enable the zoom lens 85to produce a collimated, variable-energy beam for passage into the 90°mirror 87.

In electron optics, a zoom lens accomplishes the function of changingthe energy of the electron beam while maintaining the same object andimage locations. A typical zoom lens is a three-element lens consistingof concentric hollow metal tubes in series held at voltages v1, v12, andv2, respectively. Typically, the center element is shorter in lengththan the first and third elements (e.g., see lens 84 in FIG. 18). Inthis case, v1 establishes the entrance energy, and v2 the exit energy.The ratio (v2/v1 for acceleration, and v1/v2 for deceleration) is calledthe “zoom ratio”. For a given value of v1 and v2, the value of thecenter element voltage, v12, is selected to maintain the focal lengths(and hence the object and image locations, P and Q, respectively) of thelens. Zoom lenses of this type are useable over a limited energy range(the “zoom range”). The five-element zoom lens 85 preferably employed inaccordance with the present invention and illustrated in FIG. 13 is anextension of this concept. By adding additional lens elements, thiscompound lens offers the following expanded capabilities versus athree-element lens:

-   -   1) It can be operated over an extended zoom range, e.g., 20:1        versus 5:1 for a three-element lens.    -   2) It can vary angular magnification and be operated in an        “afocal mode”, that is, by tuning the voltages so that the        electron trajectories entering the lens are parallel upon exit,        i.e., there is no real focus at the lens exit.    -   3) It can be operated as a “telescopic” lens, which produces a        real image with a well-defined value of P and Q, but with        variable linear magnification. For example, when AEL1 is a        stronger focusing lens (shorter focal length) than AEL2, M>1;        and when AEL2 is the stronger focusing lens, M<1.    -   4) The five-element lens can provide variable linear and angular        magnification while also allowing zoom control, i.e., varying        both energy and magnification.

For example, we refer to FIGS. 13A through 13D, which show thefive-element zoom lens as two three-element lenses, AEL1 and AEL2, intandem. In general, the electron beam will be at different energiesentering and exiting the zoom lens, as previously discussed. Inaddition, several modes of operation are illustrated by the figures.FIG. 13A shows the afocal mode, where the electron beam iswell-collimated exiting the lens, corresponding to an image at infinity.This mode is advantageous for collimating the beam prior to its entranceinto a turning stage, such as the 90 degree mirror 87 described withreference to FIGS. 9, 9B and 10. It is also advantageous for injecting awell-collimated beam of the desired energy into the ionization chamber,to maintain the beam substantially parallel with an elongated extractionaperture. FIG. 13B shows the beam being focused to an image with unitymagnification. This mode is desirable when a high degree of collimationis not necessary, and preservation of the beam characteristics at theobject location is desired at the image location, for example, when theobject dimension is appropriate for the size of the beam profile in theionization chamber when the zoom lens is being used primarily formodifying the energy of the electron beam. FIG. 13C shows the beam beingfocused to an image smaller than the object, which is appropriate forinjection into a mirror or into the ionization chamber whencounteraction of space charge forces in the electron beam is desired, toprevent the beam from expanding overmuch, as when the zoom action isemployed to decelarate the beam. This mode is also advantageous forproducing a narrow cross-section electron beam in conjunction with anarrow ion extraction aperture, e.g., in a medium or low current ionimplanter. FIG. 13D shows the beam being focused to an image larger thanthe object. This mode is advantageous to expand the electron beam priorto injection into the ionization volume to provide a large cross-sectionionization region, as in the case of a wide ion extraction aperture in ahigh-current ion implanter.

In conjunction with the input collimating lens 84, the lens system canexercise control of linear and angular magnification, energy, and imagelocation over a wide range, more than sufficient for the needs of thepresent invention.

The zoom lens 85 is comprised of two asymmetric einzel lenses in tandem,einzel lenses 104 and 106 in FIG. 10, and AEL1 and AEL2 in FIG. 13. Thezoom lens 85 is a five-element lens, with its center (third) element, 3Dlength, serving as an element of each of the tandem einzel lenses. FIG.13 shows an Object and Image for AEL1 (the Image is an Object for AEL2)which results in a final image at infinity, producing collimatedelectron trajectories. Zoom lens 85 is capable of being operated as anafocal lens by setting its element voltages such that the second focalpoint of AEL1 and the first focal point of AEL2 overlap. In this mode,the zoom lens 85 is telescopic; parallel electron trajectories enteringthe lens are also parallel upon exiting. In the case, however, thatV2>V3>V4, the zoom lens advantageously decelerates the electrons over awide energy range, and can still retain its telescopic properties if thevoltage differences, i.e. V23 and V34, are adjusted appropriately. Apositron lens structure of the type shown in FIG. 11, is shown in T. N.Horsky, Ph.D. thesis, Brandeis University Dept. of Physics,Semiconductor Surface Structure Determination via Low Energy PositronDiffraction: Cleavage Faces of CdSe, UMI Pub # 9010666, Chapter 3, 1988.FIG. 14, taken from that thesis, shows an example of a deceleratingoperating mode, in which lens element potentials V_(i) are expressed inkinetic energy units, i.e., kinetic energy=e|(V_(i)−V_(c))|. Thepositron beam entered the zoom lens at 1 keV, and decelerated to a beamenergy of 75 eV upon exiting AEL1 (i.e., within lens element V3). Theplot shows how V34 was varied as a function of positron final beamenergy to maintain a collimated output, for a final beam energy rangebetween 5 eV and 250 eV. The plot is indicative of plots obtainable withthe similar electron beam lens structure presented here.

In the present novel embodiment, the collimating electron lens 84 istuned in conjunction with the zoom lens 85 to vary linear magnificationas well as final electron beam energy. Thus, a variable-energy,variable-diameter electron beam can be generated with the lens systemdepicted in FIG. 10, with the advantage of copious electron productionenhanced by the acceleration geometry, while achieving lower finalelectron energy appropriate for interaction with dopant feed material,e.g., with decaborane, by use of the deceleration stage.

Prior to entering the ionization volume 16 of ionization chamber block5, the electron beam produced by the gun of FIG. 10 is turned through90°. The turning stage 87 can be of various known forms, e.g., tworelated and coaxial partial cylinders (i.e., a radial cylindricalanalyzer), formed into respectively inside and outside sides of an elbowthat bends the electron optical axis, the partial cylinder that lies onthe inside of the curved axis being maintained at a more positivepotential than the partial cylinder lying on the outside of the curvedaxis. These cooperate to turn the beam 90 degrees according to knownelectron path bending techniques.

A mirror defined by two flat or cylindrically curved plates (i.e.,either a parallel plate or cylindrical mirror analyzer) whose axis isoriented 45° from the zoom axis to result in a 90° deflection at theexit of the mirror, can also be employed to occupy a smaller spacewithin the retrofit volume. It is presently preferred, however, that thedescribed radial cylindrical analyzer be employed with the advantage ofachieving two dimensional transfommation of the beam to the new paththrough the ionization space 16 of the ionization chamber 5, thuspreserving the pre-established beam profile with high transmission.

After turning, the beam passes through a limiting aperture 10′ which isadvantageously rectangular, and enters the ionization chamber 5 via theelectron entrance port 35. Limiting aperture 10′ is constructed to bereplaceable in coordination with replacing the ion extraction aperture,typically the wider the ion extraction aperture, the larger is thecorresponding dimension of the selected electron limiting aperture 10′.

In operation with the turning mirror, at low electron energies, spacecharge forces can affect control of the electron beam. According furtherto the invention, two different modes of using the long E-Beam gun witha 90 degree turning mirror are provided, that successfully deal withthis.

E-Beam Mode 1: The deceleration capabilities of the zoom system areemployed in conjunction with the acceleration capabilities of thepreceding collimating lens, to provide an acceleration-deceleration modeof operation. For instance, the lens voltages are coordinated to causethe system to zoom down from, e.g., one keV at the entrance to the zoomsystem to 100 eV at its exit. Because the beam expands due to thedeceleration, some electrons of the beam may be lost within the mirror,but this is readily acceptable where low current, low energy injectioninto the ionization volume 16 is desired. For example, the system isoperable at currents less than 5 mA at 100 eV, or at higher energies. Asthe final energy of the electrons goes up, the electron currentincreases. The electron beam in this case can be well collimated and bealigned with a relatively small area beam dump.

E-Beam mode 2: In this case, the electrons are transported at highenergy throughout the E-Beam gun and mirror, and a deceleration stage 88is interposed between the exit aperture of the mirror and the entranceof the ionization volume 16. Because the beam is collimated at highenergy, the electron optics perform without detrimental space chargeeffect, delivering a well-collimated beam sized for the mirror.

Following the mirror, the beam is caused to decelerate abruptly as itenters the ionization chamber, to expand with the electron trajectoriesconfined to a conical, gradually expanding volume. In this case,electron currents of 20 mA or more, for example, may be obtained. As thebeam expands, since the electron trajectories remain generally straight,the beam can be intercepted by a beam dump 11 of larger area than inmode 1. Along the ionization path in this case, those electrontrajectories which diverge to pass more closely to the aperture aresomewhat offset by those which diverge further from the aperture so thattotal ions extracted along the aperture need not vary in density to anunacceptable degree along the length of the aperture. For this mode ofoperation, having the beam dump area large (with the beam dump in closeproximity to the wall of the ionization chamber to limit gasconductance), the beam dump is sized still to align with the somewhatdiverging electron paths so that substantially all electrons of theE-Beam from the mirror are intercepted by the cooled beam dump.

In the case an elongated electron gun is mounted with its axis alignedwith the ionization path through the chamber (no mirror employed),mirror loss of the beam can be avoided, and a collimated electron beam,produced as in mode 1, can be maintained through the ionization chamber,at a larger electron current.

The operation of such systems have numerous advantages under conditionsof operation appropriate to producing the ion beams illustrated indifferent circumstances such as shown in FIGS. 4D and 5. The system canproduce different size profiles of the broad area beams aligned with thebeam dump, and different electron densities suitable for respectivelydifferent situations over a wide range of preferred operation, e.g. overa zoom ratio of 15 to 1. Cost efficiency, space efficiency and thermaladvantages especially result by use of a relatively small cathode, whileachieving a relatively broad and controlled-energy beam. The system isuseful, first with respect to decaborane at electron beam energies ofbetween about 20 to 150 eV, and with many important or novel otherspecies. The different energy regimes up to, e.g. 300 or 500 eV canenable the system to operate, in broad, aligned electron beam mode withrespect to all species, (including the fluorides for small, but highlypure beams). In a specially constructed multi-mode ionization system,the system can be switched to a reflex ionizing mode for some species(e.g. hydrides and fluorides) using a confining magnetic field. It canalso be operated to produce doubly charged phosphorus or arsenic, andtriply charged species.

Electron Injection for High Current Applications

For some ion implant applications, it is desired to obtain an ioncurrent approaching the highest ion currents of which the technology iscapable. This depends critically on the value of electron beam currenttraversing the ionization chamber, since the ion current produced isroughly proportional to the value of this electron current. The electroncurrent injected into the ionization chamber is limited by the effectsof space charge forces that act on the electron trajectories within theelectron gun optics and the ionization chamber. In the space chargelimit, these forces can add an increased width to a tightly focused beamwaist produced by a lens, and can introduce an increased angulardivergence to a beam as it diverges downstream of the waist.

I note the relevance here of the principle that the maximum electroncurrent which can be transported through a tube of diameter D and lengthL can be produced by focusing the beam on a point at the center of thetube with an angle α=D/L expressed in radians. In such case, the maximumcurrent is given by:I _(max)=0.0385 V ^(3/2)α²,  (1)

where I_(max) is the electron current measured in mA, and V is thevoltage in volts corresponding to electrons of energy E=eV, where e isthe electronic charge. Also, in this example the minimum waist diameterw is given by w=0.43 D. Inserting α=15° and V=100V into equation (1)yields I_(max)=10 mA, whereas inserting α=5° and V=1000V yieldsI_(max)=106 mA.

By interpreting a as the angular divergence induced by space chargeforces, these examples demonstrate the advantage achieved by the novelembodiment of FIG. 18 which transports the space charge-limited beam athigh energy and achieves a large injected electron current at desiredlower energy.

The gun of FIG. 18 is similar to that of FIG. 10, but has importantdifferences: 1) instead of the zoom lens 85, a double-aperture lens 88is employed, which terminates at the entrance port of the ionizationchamber and 2) no mirror 87 is used, the gun being mounted coaxial withthe long axis of the ionization chamber. In a preferred embodimenthaving these features, large-diameter tubes (approximately 2.5 cmdiameter) are used to limit lens filling, and hence beam loss due toaberrations. The gun is kept short by using the collimating lens topresent the desired beam characteristics to the final double-aperturelens (DAL) for injection into the ionization chamber. Provisions aremade so that the electron gun voltages (V_(g), V_(w), V_(s), V1, V12,all referenced to the cathode voltage V_(c)) are tunable to give thebest performance in terms of beam current, angular divergence, and beamdiameter appropriate to a given application, and will operate at fixedvalues, with a beam energy E_(i) at the exit of collimating lens 84(i.e., E_(i)=e [V2−V_(c)]) between 750 eV and 1250 eV. Thus, thewide-range zoom capability provided by the gun of FIG. 10 is notrequired; the tetrode extraction gun 83 in combination with thethree-element collimating lens 84 provides sufficient flexibility tocontrol and to properly determine the electron beam characteristics. TheDAL then functions as a strongly focusing decelerating lens, with thedesired electron energy within the ionization chamber being given byE_(f)=e [V_(ch)−V_(c)], where V_(ch) is the ionization chamber potential(when V_(ch) is referenced to earth ground, it is the ion beamaccelerating potential V_(a)). For example, with E_(i)=1000 eV andE_(f)=100 eV, the DAL is a 10:1 decelerator.

In the presently preferred design of the DAL, it is comprised of twoflat plates with equal diameter circular apertures of diameter D′. Theplates (of thickness 0.1 D′) are separated by a uniform distance D′/2,and are constructed of vitrified graphite, silicon carbide, or aluminumto eliminate transition metal contamination due to beam strike on theapertures which could result if tantalum, molybdenum, or stainless steelelectrodes were used. For example, values of D′=1.2 cm±0.6 cm willaccommodate much of the useful range of this lens. Importantly, sinceone plate of the DAL is tied to V2 and the second plate is tied toV_(ch), the addition of this lens does not require a further powersupply. The DAL serves two useful purposes: 1) it accomplishesdeceleration of the electron beam, in a controlled and well-definedmanner, to the selected value of E_(f) necessary to maximize ionizationefficiency of the particular dopant feed gas of interest, and 2) itprovides strong focusing of the electron beam to counteract space chargeeffects which would otherwise dominate the spreading of the electrontrajectories within the ionization volume.

I further recognize the advantage here of the principle that, in orderto maximize the electron current through a tube, the beam should befocused at the center of the tube length. According to the presentinvention, when injecting the space charge-limited electron beam intothe field-free volume of the ionization chamber, the spreading of thebeam is minimized by focusing the beam at the center of the volume'slength. In the case of an ion source according to FIG. 18, the nominalfocus is located a distance of about 4 cm from the principal plane ofthe DAL. The optics for this are shown in more detail in FIG. 18A. Anobject O′ is presented to the DAL by the upstream lens, and acorresponding image I′ of this object is produced by the DAL. The valuesused for this model are: V2/V_(ch)=10, D′=1.27 cm, object distanceP=4.8D′, image distance Q=3D′, linear magnification M=1.0 (taken from E.Harting and F. H. Read, Electrostatic Lenses, Elsevier, New York, 1976).Thus, in this embodiment the electron beam is focused to an image point3.8 cm from the principal plane of the DAL, approximately in the centerof the length of the ionization chamber. By varying the lens ratioV2/V_(ch) and/or changing the position of the object, the location ofthis image point can be moved to optimize the performance of the ionsource in relation to other operating parameters (for example, the imagecan be moved further downstream, so that the minimum waist diameter ofthe beam, i.e. the circle of least confusion, falls near the center ofthe chamber). The maximum extent of the space charge spreading of thebeam may be estimated through use of equation (1), rearranging it asbelow:D=5.1L I _(max) ^(1/2) V ^(−3/4)  (2)

where D is the diameter intercepted by the electron flux, and L is thelength of the ionization chamber (approximately 7.6 cm). Substitution ofI_(max)=20 mA and V=100V yields D=5.5 cm, as does substitution of 1mps=40 mA and V=168V. Indeed, in practice, the space charge spreading inthe ionization chamber will be less than approximated by equation (2)due to the space-charge compensation provided by the positively-chargedions which are abundantly present in the ionization volume. FIG. 18 andFIG. 18A employ an enlarged electron exit port and beam dump 36 tointercept the vast majority of the electrons in the beam. By keeping theseparation between the beam dump 11 and the ionization chamber small,the gas flow out of the ionization volume through the exit port 36 canbe small.

Several advantageous features of the ionization chamber and ionextraction aperture are also shown in FIG. 18A: 1) a counterbore isprovided in the chamber wall to receive the thin aperture plate in sucha way as to maintain a uniformly flat profile, to establish a uniformelectric field between the aperture plates; 2) the ion extractionaperture 37′ is moved closer to the center of the chamber (by up toabout 8 mm, or 25% of the width of the chamber) for more efficientremoval of ions by the extraction field of the extraction optics, and ashorter ion path through the ionization volume which reduces theprobability of ion-neutral gas collisions, resulting in an asymmetriclocation in the chamber of the electron entrance-exit axis; 3) the ionextraction aperture plate is biased to a negative voltage V_(E) (where−25V<V_(E)<0V) with respect to the ionization chamber to furtherincrease the drift velocity of the ions, and hence the maximumobtainable current in the resulting ion beam.

Referring to the embodiment of FIGS. 19-19B, biasing of the apertureplate is accomplished by forming it of an insulating material such asboron nitride, coating the exterior and interior surfaces which areexposed to the ions with an electrically conductive material such asgraphite, and electrically biasing the conductor.

In other embodiments insulator standoffs are employed, see FIG. 19C, tojoin the electrically conductive extraction aperture plate to thechamber while maintaining its electrical independence. In embodiments ofthis feature, gas loss from the ionization chamber at the edges of theaperture plate can be minimized by interfitting conformation of theedges of the electrically isolated aperture plate and the body of theionization chamber (involuted design) to effect labyrinth seal effectssuch as described in relation to FIG. 4B.

In accordance with the embodiment of FIGS. 20A, B and C, an electricallyconductive aperture plate insert is mounted in an electricallyinsulating frame which holds the aperture plate in place, and providesan electrical contact to the insert.

The embodiment facilitates change of aperture plates in accordance withchanges of the type of implant run. In some embodiments thermoelectriccoolers may be associated with the aperture plates to keep them fromover-heating. In other embodiments, an extension of cooled frame 10 or aseparate cooled mounting frame is employed to support the apertureplate.

Retrofit Embodiment of High Current Source

FIG. 18B shows the introduction of the embodiment of FIG. 18 and FIG.18A into the ion source housing of a retrofitted implanter. Preferablythe electron gun is mounted at the top, as shown. To implement thisgeometry into an existing implanter, a new ion source housing isprovided, constructed in accordance with typical Bernas ion sourceconsiderations, (it can receive a Bernas ion source if ever desired),but the housing is modified at the top to receive the electron gun. Inanother case the existing ion source housing is modified, e.g. by theremoval of the magnet coils 54 and the insertion of a vacuum port at thetop of the housing to receive the flange-mounted, vertical electron gunassembly.

Since the implementation of an external, axial magnetic field can incertain cases be useful, a small pair of magnet coils is provided, asalso shown in FIG. 18B. The electron gun as shown here, is mountedcoaxially within one of those coils in a space efficient and uniquelycooperative arrangement.

When these magnet coils are energized, the resultant axial magneticfield can confine the primary electron beam (both within the electrongun and in the ionization chamber) to a narrowed cross-section, toreduce the spreading of the electron beam profile due to space charge,and increasing the maximum amount of useful electron current which canbe injected into the ionization volume. For example, a magnetic fluxdensity of 70 Gauss will act to confine 100 eV electrons within theionization volume to a column diameter of about 1 cm. Since the electronemitter of this long electron gun is remote from the ionization chamber,it will not initiate an arc discharge, while, depending on the strengthof the external magnetic field, it will provide a low-density plasmawithin the ionization region. By controlling this plasma to a low value,multiple ionizations induced by secondary electron collisions with theions can be controlled to acceptable levels in certain instances.Furthermore, it is realized that the presence of the low-density plasma,in some instances, can enhance the space charge neutrality of theionization region, and enable higher ion beam currents to be realized.

In a multi-mode embodiment, larger magnets are employed in therelationship shown in FIG. 18B to enable larger magnetic fields to beemployed when operating in reflex mode, or when a Bernas arc dischargesource is desired to be used.

Universal Ion Source Controller

A universal controller for the ion source of the invention uniquelyemploys the user interface that is used with arc discharge ion sourcessuch as the Bernas and Freeman types. FIG. 15 shows, in diagrammaticform, a typical control system 200 for operating a Bernas type ionsource. The operator for such existing machines programs the implanterthrough an Operator Interface 202 (OI), which is a set of selectablegraphical user interfaces (GUI's) that are selectively viewed on acomputer screen. Certain parameters of the implanter are controlleddirectly from the OI, by either manually inputting data or by loading apredefined implant recipe file which contains the desired parametersthat will run a specific implant recipe. The available set of GUI'sincludes controls and monitoring screens for the vacuum system, waferhandling, generation and loading of implant recipes, and ion beamcontrol.

In many implanter systems, a predetermined set of ion source parametersis programmable through the Beam Control Screen of the OI represented inFIG. 15, including user-accessible setpoint values for Arc Current, ArcVoltage, Filament Current Limit, and Vaporizer Temperature. In additionto these setpoints, the actual values of the same parameters (forexample, as indicated by the power supply readings) are read back anddisplayed to the operator on the OI by the control system.

Many other parameters that relate to the initial set up of the beam fora given implant are programmed and/or displayed through the Beam ScreenGUI, but are not considered part of the operator's ion source control.These include beam energy, beam current, desired amount of the ion,extraction electrode voltages, vacuum level in the ion source housing,etc.

As indicated in FIG. 15, a dedicated Ion Source Controller 204 reads andprocesses the input (setpoint) values from the OI, provides theappropriate programming signals to the stack of power supplies 206, andalso provides read backs from the power supplies to the OI. A typicalpower supply stack 206 shown in FIG. 15, includes power supplies for theArc, Filament, and Vaporizer Heater, power supplies 208, 210 and 212,respectively. The programming and power generation for the Source MagnetCurrent may be provided in the screen, but is typically providedseparate from the Ion Source Controller in many machines of thepresently installed fleet.

FIG. 15 a shows the same elements as FIG. 15, but for a Bernas-style ionsource of the kind which uses an indirectly-heated cathode (IHC). FIG.15 a is identical to FIG. 15, except for the addition of a Cathode powersupply 211, and its read back voltage and current. The additional powersupply is necessary because the IHC (indirectly heated cathode element)is held at a positive high voltage with respect to the filament, whichheats the IHC by electron bombardment to a temperature sufficient thatthe IHC emits an electron current equal to the Arc Current setpointvalue provided through the OI. The arc control is accomplished through aclosed-loop control circuit contained within the Ion Source Controller.

FIG. 16 shows diagrammatically the functional design of the ElectronBeam Ion Source Controller 220 of the present invention. Control ofelectron current from the electron gun directed to the beam dump 36 isaccomplished by a closed-loop servo circuit within the controller 220which adjusts the electron emitter temperature and the electron gun gridpotential to maintain the desired electron current setpoint. TheController 220 is designed to be retrofittable into a typical existingimplanter, both functionally and mechanically, and to do so withessentially no change to the controls software of the implanter. Inorder to achieve mechanical retrofittability, the Controller electronics220 and Ion Source Power Supplies 207 occupy a similar physical volumein the gas box as did the existing Bernas Ion Source Controller 204 andPower Supplies 206. In order to preserve the integrity of theimplanter's existing controls software, the Controller 220 isconstructed to accept the existing inputs from the O1202 and to providethe read backs expected by the OI. Thus, the operator can program theIon Source I of the present invention from the OI in the manner to whichthe operator has long been accustomed, without change. Thisfunctionality is accomplished by a configurable Universal Translatorcircuit board 222 contained within the Controller 220, which acceptsanalog or digital inputs from the OI 202, and converts these inputs tothe appropriate programming signals for the control of the Electron Beamof the ion source 1 of the present invention. This signal processingincludes, as appropriate, digital-to-analog conversion, 16 bitdigital-to-20 bit-digital conversion, analog-to-digital conversion,signal inversion, and multiplication of the signal by a scale factor,for example, depending upon the type and manufacturer of the installedion implanter into which the broad, aligned electron beam ion source isto be retrofit. In like manner, the configurable Universal Translator222 then processes the read back signals provided by the Electron BeamPower Supplies 207, and reports back to the OI 220 in the digital oranalog format expected by the OI. The configurable Universal Translator222 is also configurable to the specific number and kinds of outputsrequired by the installed implanter control system, for example todifferentiate between a Bernas source and an IHC Bernas source, whichrequires extra read back channels for cathode voltage and current and adifferent scale factor for the cathode current limit setpoint vis-à-visthe Bernas and Freeman ion sources. The configurable UniversalTranslator 222 accomplishes this by substituting the control variablesas indicated in FIG. 16, and as also shown in Table II below, for thecase of a directly heated cathode electron gun in the E-Beam ion sourceof the invention. In the case of the system being retrofit to replace anIHC Bernas source, the two variables in the screen related to cathodevoltage and filament current are assigned the optional values of anodevoltage and cathode heating current. In the case of an indirectly heatedelectron source being used in a retrofit E-Beam ion source according tothe invention, the values of its cathode voltage and heating filamentcurrent can be substituted for the optional values listed. TABLE IIControls Variables OI Setpoint OI Setpoint E-Beam E-Beam OI Read Back OIRead Back (Bernas) (IHC Bernas) Setpoint Read Back (Bernas) (IHC Bernas)Filament Cathode Emission Emission Filament Cathode Current LimitCurrent Limit Current Limit Current Current Current Arc Current ArcCurrent Beam Dump Beam Dump Arc Current Arc Current Current Current ArcVoltage Arc Voltage E-Beam Cathode Arc Voltage Arc Voltage EnergyVoltage Vaporizer Vaporizer Vaporizer Vaporizer Vaporizer VaporizerTemperature Temperature Temperature Temperature Temperature Temperature— — — Anode* — Cathode Voltage Voltage Cathode* Filament Heating CurrentCurrent*optional

Additional electron beam control settings, for example many of the lensvoltages shown in FIG. 11, are not accessible to the user through theOI, but must be preset at the Controller. Some of these voltage settingsare accessible manually through potentiometers on the front panel (whichprovides visual read backs through panel-mounted meters while others(for example, V_(g) and V_(w) of the long extraction gun and V3 and V34of the zoom lens) are automatically set through firmware-based lookuptables resident in the Controller electronics.

In general, the arc control of Bernas, Freeman, and IHC Bernas sourcesare accomplished through similar means, namely by on-board closed-loopcontrol circuits contained within the Ion Source Controller. In order tophysically retrofit the ion source of an existing ion implanter with anion source of the present invention, the original ion source is removedfrom the source housing of the implanter, the power cables are removed,and the Ion Source Controller 204 and the power supplies 206 or 206 ¹,i.e. the Filament Power Supply, Vaporizer Power Supply, Arc PowerSupply, and Cathode Power Supply (if present) are removed from the gasbox of the implanter. The Electron Beam Ion Source 1 of the presentinvention is inserted into the retrofit volume of the implanter, and theElectron Beam Ion Source Controller 220 and associated Power Supplies207 are inserted into the vacated volume of the gas box. A new set ofcables is connected. The desired mechanical configuration of the ionsource is prepared prior to installation into the source housing of theimplanter. For example, for decaborane production, a large width ionextraction aperture and a large dimension limiting aperture at the exitof the electron gun can be installed, to provide a large ionizationvolume. Additionally, if the implanter has installed a variable-widthmass resolving aperture 44, the width of that aperture may be increasedin order to pass a larger mass range of decaborane ions. Otherwise, theset-up proceeds in a conventional manner, modified according to thevarious features that are explained in the present text.

In addition to the electron beam controls that have just been explained,a temperature control mechanism is provided for the vaporizer 2. Thevaporizer is held at a well-defined temperature by a closed-looptemperature control system within the Controller 220. As has beenexplained above, the closed-loop temperature control system incorporatesPID (Proportional Integral Differential) control methodology, as isknown in the art. The PID controller accepts a temperature setpoint andactivates a resistive heater (which is mounted to a heater plate incontact with the water bath (see FIG. 3), or in heat transferrelationship with the mass of the vaporizer body 29 (FIG. 3A) to reachand maintain its setpoint temperature through a thermocouple read backcircuit. The circuit compares the setpoint and read back values todetermine the proper value of current to pass through the resistiveheater. To ensure good temperature stability, a water-cooled heatexchanger coil 21 is immersed in the water bath (in the case of thewater-cooled vaporizer of FIG. 3), or a thermoelectric (TE) cooler 30(in the embodiment of a solid metal vaporizer of FIG. 3A), or aheat-exchanger coil surrounded by heat-conducting gas (in the embodimentof a vaporizer utilizing pressurized gas to accomplish thermalconduction between the various elements as in FIG. 3F) to continuallyremove heat from the system, which reduces the settling time of thetemperature control system. Such a temperature control system is stablefrom 20C to 200C. In this embodiment, the flow of gas from the vaporizerto the ionization chamber is determined by the vaporizer temperature,such that at higher temperatures, higher flow rates are achieved. Asimilar temperature control system can be employed to control thetemperature of conductive block 5 a of FIG. 3E or 9B.

As has also previously been explained, in another embodiment a differentvaporizer PID temperature controller is employed. In order to establisha repeatable and stable flow, the vaporizer PID temperature controllerreceives the output of an ionization-type pressure gauge which istypically located in the source housing of commercial ion implanters tomonitor the sub-atmospheric pressure in the source housing. Since thepressure gauge output is proportional to the gas flow into the ionsource, it output can be employed as the controlling input to the PIDtemperature controller. The PID temperature controller can subsequentlyraise or diminish the vaporizer temperature, to increase or decrease gasflow into the source, until the desired gauge pressure is attained.Thus, two useful operating modes of a PID controller are defined:temperature-based, and pressure-based.

Referring to FIG. 16B, in another embodiment, these two approaches areuniquely combined such that short-term stability of the flow rate fromthe vaporizer is accomplished by temperature programming alone, whilelong-term stability of the flow rate is accomplished by adjusting thevaporizer temperature through software to meet a pressure setpoint whichis periodically sampled. The advantage of such a combined approach isthat, as the solid feed material is consumed by vaporization, thetemperature is slowly raised by software control to compensate for thesmaller flow rates realized by the reduced surface area of the materialpresented to the vaporizer, in accordance with pressure sensed by thepressure gauge in the source housing. In FIG. 16B the ionization gauge300 which monitors pressure within the ion source housing is the sourceof an analog pressure signal applied to an analog to digital converter,ADC. The digital output is directed to the CPU which, under softwarecontrol, evaluates the drift of pressure over time, and introduces agradual change in temperature setting to stabilize the pressure in itsoptimal range.

In the embodiments of. FIGS. 3 and 3A, temperature of the ionizationchamber is controlled by the temperature of the vaporizer. Temperaturecontrol for the embodiments of FIGS. 3E, 9B and 18B is achieved by aseparate temperature sensing and control unit to control the temperatureof the metal heat sink by use of a heat transfer medium orthermoelectric coolers or both.

Calculations of Expected Ion Current

The levels of ion current production that can be achieved with this newion source technology are of great interest. Since the ion source useselectron-impact ionization by energetic primary electrons in awell-defined sizeable ionization region defined by the volume occupiedby the broad electron beam in traversing the ionization chamber, its ionproduction efficiency can be calculated within the formalism of atomicphysics:I=I _(0[)1−exp{−n l s}],  (3)

where I₀ is the incident electron current, I is the electron currentaffected by a reaction having cross section s, n is the number densityof neutral gas molecules within the ionization volume, and l is the pathlength. This equation can be expressed as follows:f=1−exp{−L s pl},  (4)

where f is the fraction of the electron beam effecting ionization of thegas, L is the number density per Torr of the gas molecules at 0C(=3.538×10¹⁶ Torr⁻¹ cm⁻³), s is the ionization cross section of thespecific gas species in cm², and pl is the pressure-path length productin Torr-cm.

The peak non-dissociative ionization cross section of decaborane has notbeen published, so far as the inventor is aware. However, it should besimilar to that of hexane (C₆H₁₄), for example, which is known to beabout 1.3×10¹⁵ cm². For an ion source extraction aperture 5 cm long andan ionization chamber pressure of 2×10⁻³ Torr, equation (2) yieldsf=0.37. This means that under the assumptions of these calculationsdescribed below, 37% of the electrons in the electron current producedecaborane ions by single electron collisions with decaborane molecules.The ion current (ions/sec) produced within the ionization volume can becalculated as:I _(ion) =f I _(el,)  (5)

where I_(ion) is the ion current, and I_(el) is the electron currenttraversing the ionization volume. In order to maximize the fraction ofion current extracted from the ion source to form the ion beam, it isimportant that the profile of the electron beam approximately matches inwidth the profile of the ion extraction aperture, and that the ions areproduced in a region close to the aperture. In addition, the electroncurrent density within the electron beam should be kept low enough sothat the probability of multiple ionizations, not taken into account byequations (3) and (4), is not significant.

The electron beam current required to generate a beam of decaborane ionscan be calculated as:I _(el) =I _(ion) /f,  (6)

Given the following assumptions: a) the decaborane ions are producedthrough single collisions with primary electrons, b) both the gasdensity and the ion density are low enough so that ion-ion andion-neutral charge-exchange interactions do not occur to a significantdegree, e.g., gas density <10 ¹⁴ cm⁻³ and ion density <10¹¹ cm⁻³,respectively, and c) all the ions produced are collected into the beam.For a 1 mA beam of decaborane ions, equation (6) yields I_(el)=2.7 mA.Since electron beam guns can be constructed to produce electron currentdensities on the order of 20 mA/cm², a 2.7 mA electron beam currentappears readily achievable with the electron beam gun designs describedin this application.

The density of primary electrons n_(e) within the ionization volume isgiven by:n _(e) =J _(e) /ev _(e),  (7)

where e is the electronic charge (=1.6×10⁻¹⁹C), and v_(e) is the primaryelectron velocity. Thus, for a 100 eV, 20 mA electron beam of 1 cm²cross-sectional area, corresponding to a relatively wide ion extractionaperture as illustrated in FIG. 4F, equation (7) yields n_(e)≈2×10¹⁰cm⁻³. For a narrow extraction aperture, as illustrated in FIG. 5, a 100eV, 20 mA of 0.4 cm² cross-sectional area would provide an electrondensity n_(e)≈5×10¹⁰ cm⁻³. Since the ion density, n_(i), within theionization volume will likely be of the same order of magnitude asn_(e), it is reasonable to expect n_(i)<10¹¹ cm⁻³. It is worth notingthat since n_(e) and n_(i) are expected to be of similar magnitude, somedegree of charge neutrality is accomplished within the ionization volumedue to the ionizing electron beam and ions being of opposite charge.This measure of charge neutrality helps compensate the coulomb forceswithin the ionization volume, enabling higher values of n_(e) and n_(i),and reducing charge-exchange interactions between the ions.

An important further consideration in determining expected extractioncurrent levels from the broad, collimated electron beam mode is theChild-Langmuir limit, that is, the maximum space charge-limited ioncurrent density which can be utilized by the extraction optics of theion implanter. Although this limit depends somewhat on the design of theimplanter optics, it can usefully be approximated as follows:J _(max)=1.72(Q/A)^(1/2) U ^(3/2) d ⁻²,  (8)

where J_(max) is in mA/cm², Q is the ion charge state, A is the ion massin amu, U is the extraction voltage in kV, and d is the gap width in cm.For B₁₀H_(x) ⁺ ions at 117 amu extracted at 5 kV from an extraction gapof 6 mm, equation (6) yields J_(max)=5 mA/cm². If we further assume thatthe area of the ion extraction aperture is 1 cm², we deduce aChild-Langmuir limit of 5 mA of B₁₀H_(x) ⁺ ions at 5 keV, whichcomfortably exceeds the extraction requirements detailed in the abovediscussion.

Ion Extraction Aperture Considerations for the Broad, Aligned BeamElectron Gun Ion Source

It is realized, that for the broad electron beam ion source of thepresent invention, it is possible to employ a larger width ionextraction aperture than typically employed with high current Bernas arcdischarge sources. Ion implanter beam lines are designed to image theextraction aperture onto the mass resolving aperture, which is sized toboth achieve good transmission efficiency downstream of the massresolving aperture, and also to maintain a specified mass resolution R(≡M/ΔM, see discussion above). The optics of many high-current beamlines employ unity magnification, so that, in the absence ofaberrations, the extent of the ion extraction aperture as imaged ontothe resolving aperture is approximately one-to-one, i.e., a massresolving aperture of the same width as the ion extraction aperture willpass nearly all the beam current of a given mass-to-charge ratio iontransported to it. At low energies, however, space charge forces andstray electromagnetic fields of a Bernas ion source cause both anexpansion of the beam as imaged onto the mass resolving aperture, andalso a degradation of the mass resolution achieved, by causingsignificant overlap of adjacent beams of different mass-to-charge ratioions dispersed by the analyzer magnet.

In contrast, in the ion source of the present invention, the absence ofa magnetic field in the extraction region, and the lower total ioncurrent level desired, e.g. for decaborane relative say to boron,uniquely cooperate to produce a much improved beam emittance with loweraberrations. For a given mass resolving aperture dimension, this resultsin higher transmission of the decaborane beam through the mass resolvingaperture than one might expect, as well as preserving a higher R.Therefore, the incorporation of a wider ion extraction aperture may notnoticeably degrade the performance of the beam optics, or the massresolution of the implanter. Indeed, with a wider aperture operation ofthe novel ion source can be enhanced, 1) because of the greater opennessof the wider aperture, the extraction field of the extraction electrodewill penetrate farther into the ionization volume of the ionizationchamber, improving ion extraction efficiency, and 2) it will enable useof a relatively large volume ionization region. These cooperate toimprove ion production and reduce the required density of ions withinthe ionization volume to make the ion source of the invention productionworthy in many instances.

Care can be taken, however, not to negatively impact the performance ofthe extraction optics of the implanter. For example, the validity ofequation (8) can suffer if the extraction aperture width w is too largerelative to the extraction gap d. By adding the preferred constraintthat w is generally equal to or less than d, then for the example givenabove in which d=6 mm, one can use a 6 mm aperture as a means toincrease total extracted ion current.

For retrofit installations, advantage can also be taken of the fact thatmany installed ion implanters feature a variable-width mass resolvingaperture, which can be employed to open wider the mass resolvingaperture to further increase the current of decaborane ions transportedto the wafer. Since it has been demonstrated that in many cases it isnot necessary to discriminate between the various hydrides of theB₁₀H_(x) ⁺ ion to accomplish a well-defined shallow p-n junction (sincethe variation in junction depth created by the range of hydride massesis small compared to the spread in junction location created by borondiffusion during the post-implant anneal), a range of masses may bepassed by the resolving aperture to increase ion yield. For example,passing B₁₀H_(x) ⁺ through B₁₀H_(x) ⁺ (approximately 113 amu through 120amu) in many instances will not have a significant process impactrelative to passing a single hydride such as B₁₀H₈ ⁺, and yet enableshigher dose rates. Hence, a mass resolution R of 16 can be employed toaccomplish the above example without introducing deleterious effects.Decreasing R through an adjustable resolving aperture can be arrangednot to introduce unwanted cross-contamination of the other species(e.g., As and P) which may be present in the ion source, since the massrange while running decaborane is much higher than these species. In theevent of operating an ion source whose ionization chamber has beenexposed to In (113 and 115 amu), the analyzer magnet can be adjusted topass higher mass B₁₀H_(x) ⁺ or even lower mass B₉H_(x) ⁺ molecular ions,in conjunction with a properly sized resolving aperture, to ensure thatIn is not passed to the wafer.

Furthermore, because of the relatively high concentration of the desiredion species of interest in the broad electron beam ion source, and therelatively low concentration of other species that contribute to thetotal extracted current (reducing beam blow-up), then, though theextracted current may be low in comparison to a Bernas source, arelatively higher percentage of the extracted current can reach thewafer and be implanted as desired.

Benefits of Using Hydride Feed Gases, etc.

It is recognized that the beam currents obtainable with the broadelectron beam ion source described can be maximized by using feed gasspecies which have large ionization cross sections. Decaborane fallsinto this category, as do many other hydride gases. While arcplasma-based ion sources, such as the enhanced Bernas source,efficiently dissociate tightly-bound molecular species such as BF₃, theytend to decompose hydrides such as decaborane, diborane, germane, andsilane as well as trimethyl indium, for example, and generally are notproduction-worthy with respect to these materials. It is recognized,according to the invention, however that these materials and otherhydrides such as phosphene and arsine are materials well-suited to theion source described here (and do not present the fluorine contaminationproblems encountered with conventional fluorides). The use of thesematerials to produce the ion beams for the CMOS applications discussedbelow, using the ion source principles described, is therefore anotherimportant aspect of the present invention.

For example, phosphene can be considered. Phosphene has a peakionization cross section of approximately 5×10⁻¹⁶ cm². From thecalculations above, equation (6) indicates that a broad, collimatedelectron beam current of 6.2 mA should yield an ion current of 1 mA ofAsH_(x) ⁺ ions. The other hydrides and other materials mentioned haveionization cross sections similar to that of phosphene, hence under theabove assumptions, the ion source should produce 1 mA for all thespecies listed above with an electron beam current of less than 7 mA. Onthe further assumption that the transmission of the implanter is only50%, the maximum electron beam current required would be 14 mA, which isclearly within the scope of electron beam current available from currenttechnology applied to the specific embodiments presented above.

It follows from the preceding discussion that ion currents as high as2.6 mA can be transported through the implanter using conventional ionimplanter technology. According to the invention, for instance, thefollowing implants can be realized using the indicated feed materials inan ion source of the present invention: Low energy boron: vaporizeddecaborane (B₁₀H₁₄) Medium energy boron: gaseous diborane (B₂H₆)Arsenic: gaseous arsine (AsH₃) Phosphorus: gaseous phosphene (PH₃)Indium: vaporized trimethyl indium In(CH₃)₃ Germanium: gaseous germane(GeH₄) Silicon: gaseous silane (SiH₄).

The following additional solid crystalline forms of In, most of whichrequire lower vaporizer temperatures than can be stably and reliablyproduced in a conventional ion source vaporizer such as is in common usein ion implantation, can also be used in the vaporizer of the presentinvention to produce indium-bearing vapor: indium fluoride (InF₃),indium bromide (InBr), indium chloride (InCl and InCl₃), and indiumhydroxide {In(OH)₃}. Also, antimony beams may be produced using thetemperature-sensitive solids Sb₂O₅, SbBr₃ and SbCl₃ in the vaporizer ofthe present invention.)

In addition to the use of these materials, the present ion sourceemploying the broad, aligned electron beam in a non-reflex mode ofoperation can ionize fluorinated gases including BF₃, AsF₅, PF₃, GeF₄,and SbF₅, at low but sometimes useful atomic ion currents through singleionizing collisions. The ions obtainable may have greater ion purity(due to minimization of multiple collisions), with lessened space chargeproblems, than that achieved in the higher currents produced by Bernassources through multiple ionizations. Furthermore, in embodiments of thepresent invention constructed for multimode operation, all of theforegoing can be achieved in the broad, aligned electron beam mode,without reflex geometry or the presence of a large magnetic confiningfield, while, by switching to a reflex geometry and employing a suitablemagnetic field, a level of arc plasma can be developed to enhance theoperation in respect of some of the feed materials that are moredifficult to ionize or to obtain higher, albeit less pure, ion currents.

To switch between non-reflex and reflex mode, the user can operatecontrols which switch the beam dump structure from a positive voltage(for broad, aligned electron beam mode) to a negative voltageapproaching that of the electron gun, to serve as a repeller(anticathode) while also activating the magnet coils 54. The coils,conventionally, are already present in the implanters originallydesigned for a Bernas ion source, into which the present ion source canbe retrofit. Thus a multi-mode version of the present ion source can beconverted to operate with an arc plasma discharge (in the case of ashort electron gun in which the emitter is close to the ionizationvolume as in FIGS. 4A-4D), in a manner similar to a Bernas source of thereflex type, or with a plasma without an active arc discharge if theemitter is remote from the ionization volume. In the embodimentdescribed previously (and also described in FIGS. 18, 18 a and 18 b) theexisting magnet coils can be removed and modified magnet coils providedwhich are compatible with the geometry of a retrofitted, long,direct-injection electron gun. When these magnet coils are energized,the resultant axial magnetic field can confine the primary electron beam(both within the electron gun and in the ionization chamber) to anarrower cross-section, reducing the spreading of the electron beamprofile due to space charge, and increasing the maximum amount of usefulelectron current which can be injected into the ionization volume. Sincethe electron emitter of this embodiment is remote from the ionizationchamber, it will not initiate an arc discharge, but depending on thestrength of the external magnetic field, will provide a low-densityplasma within the ionization region. If the plasma density is lowenough, multiple ionizations induced by secondary electron collisionswith the ions should not be significant; however, the presence of alow-density plasma may enhance the space charge neutrality of theionization region, enabling higher ion beam currents to be realized.

Benefits of Using Dimer-Containing Feed Materials

The low-temperature vaporizer of the present invention canadvantageously use, in addition to the materials already mentioned,other temperature-sensitive solid source materials which cannot reliablybe used in currently available commercial ion sources due to their lowmelting point, and consequently high vapor pressure at temperaturesbelow 200C. I have realized that solids which contain dimers of thedopant elements As, In, P, and Sb are useful in the ion source andmethods presented here. In some cases, vapors of thetemperature-sensitive dimer-containing compounds are utilized in theionization chamber to produce monomer ions. In other cases, the crackingpattern enables production of dimer ions. Even in the case ofdimer-containing oxides, in certain cases, the oxygen can besuccessfully removed while preserving the dimer structure. Use of dimerimplantation from these materials can reap significant improvements tothe dose rate of dopants implanted into the target substrates.

By extension of equation (8) which quantifies the space charge effectswhich limit ion extraction from the ion source, the following figure ofmerit which describes the easing of the limitations introduced by spacecharge in the case of molecular implantation, relative to monatomicimplantation, can be expressed:Δ=n(V ₁ /V ₂)^(3/2)(m ₁ /m ₂)^(−1/2)  (9)

where A is the relative improvement in dose rate achieved by implantinga molecular compound of mass m, and containing n atoms of the dopant ofinterest at an accelerating potential V₁, relative to a monatomicimplant of an atom of mass m₂ at an accelerating potential V₂. In thecase where V₁ is adjusted to give the same implantation depth into thesubstrate as the monomer implant, equation (9) reduces to Δ=n². Fordimer implantation (e.g., As₂ versus As), Δ=4. Thus, up to a fourfoldincrease in dose rate can be achieved through dimer implantation. TableIa below lists materials suitable for dimer implantation as applied tothe present invention. TABLE Ia Compound Melting Pt (deg C.) DopantPhase As₂O₃ 315 As₂ Solid P₂O₅ 340 P₂ Solid B₂H₆ — B₂ Gas In₂(SO₄)₃ ×H₂O 250 In₂ Solid Sb₂O₅ 380 Sb₂ Solid

Where monomer implantation is desired, the same dimer-containing feedmaterial can advantageously be used, by adjusting the mode of operationof the ion source, or the parameters of its operation to sufficientlybreak down the molecules to produce useful concentrations of monomerions. Since the materials listed in Table Ia contain a high percentageof the species of interest for doping, a useful beam current of monomerdopant ions can be obtained.

Use of the Ion Source in CMOS Ion Implant Applications

In present practice, ion implantation is utilized in many of the processsteps to manufacture CMOS devices, both in leading edge and traditionalCMOS device architectures. FIG. 17 illustrates a generic CMOSarchitecture and labels traditional implant applications used infabricating features of the transistor structures (from R. Simonton andF. Sinclair, Applications in CMOS Process Technology, in Handbook of IonImplantation Technology, J. F. Ziegler, Editor, North-Holland, New York,1992). The implants corresponding to these labeled structures are listedin Table I below, showing the typical dopant species, ion energy, anddose requirements which the industry expects to be in production in2001. TABLE I Energy Label Implant Specie (keV) Dose (cm⁻²) A NMOSsource/drain As 30-50 1e15-5e15 B NMOS threshold adjust (V_(t)) P 20-802e12-1e13 C NMOS LDD or drain extension P 20-50 1e14-8e14 D p-well (tub)structure B 100-300 1e13-1e14 E p-type channel stop B 2.0-6   2e13-6e13F PMOS source/drain B 2.0-8   1e15-6e15 G PMOS buried-channel V_(t) B10-30 2e12-1e13 H PMOS punchthrough P  50-100 2e12-1e13 suppression In-well (tub) structure P 300-500 1e13-5e13 J n-type channel stop As40-80 2e13-6e13 K NMOS punchthrough B 20-50 5e12-2e13 suppression L PMOSLDD or drain extension B 0.5-5   1e14-8e14 M Polysilicon gate doping As,B 2.0-20  2e15-8e15

In addition to the implants listed in Table I, recent processdevelopments include use of C implants for gettering, use of Ge or Sifor damage implants to reduce channeling, and use of medium-current Inand Sb. It is clear from Table I that, apart from creating thesource/drains and extensions, and doping the polysilicon gate, all otherimplants require only low or medium-dose implants, i.e. doses between2×10¹² and 1×10¹⁴ cm⁻². Since the ion current required to meet aspecific wafer throughput scales with the desired implanted dose, itseems clear that these low and medium-dose implants can be performedwith the broad, aligned electron beam ion source of the presentinvention at high wafer throughput with ion beam currents below 1 mA ofP, As, and B. Further, of course, the decaborane ion currents achievableaccording to the present invention should enable producing the p-typesource/drains and extensions, as well as p-type doping of thepolysilicon gates.

It is therefore believed that the broad, aligned electron beam ionsource described above enables high wafer throughputs in the vastmajority of traditional ion implantation applications by providing abeam current of 1 mA of B₁₀H₁₄, As, P, and B or B₂. The addition of Ge,Si, Sb, and In beams in this current range, also achievable with thepresent invention, will enable more recent implant applications notlisted in Table 1.

1. An ion source for use with an ion implant device, the ion sourcecomprising: An ionization chamber defined by a plurality of side wallsdefining an ionization volume, one of said sidewalls including an ionextraction aperture for enabling an ion beam to be extracted from saidionization chamber along a predetermined axis defining an ion beam axis;an electron beam source and an aligned beam receptor configured relativeto said ionization chamber to cause an electron beam to be directedacross the ionization volume of said ionization chamber in a directiongenerally perpendicular to said ion beam axis for ionizing gas in theionization chamber by direct electron impact ionization by energeticelectrons; and a gas source in fluid communication with said ionizationchamber. 2-65. (canceled)